This is a list, with brief descriptions of those immortals who did not even make it into the "Other Immortals" section of WotI. The list is by no means comprehensive as I do not own all Mystaran products.
The information presented contains a lot of my own ideas, and are generally not based on the mythological views of these beings (as I dislike this practice), so any disputes as to my interpretations or views are welcomed. Also I have changed some of the information presented in some of the sources.
Immortals of the Known World - Other Immortals
by Shawn StanleyFather Time: Hierarch, Sphere of Time. This immortal is merely Khoronus under a different name. (Original Immortals set, DM's Guide, page 41)
Grammaton: Hierarch, Sphere of Thought. This immortal is the identity by which Noumena is known to sphinxes. If this is taken to be so then it implies that sphinxes have only existed since 500 BC at the earliest, as this is when Noumena attained immortality. Noumena created sphinxes to help mortals become more like him, puzzle solvers. Even if Noumena is not Grammaton then it seems clear that Grammaton is an immortal of the Sphere of Thought. (PC2, page 25)
Grim Reaper: Hierarch, Sphere of Time. This immortal is actually Thanatos as he is known in some areas. Thanatos is not a Hierarch of the Sphere of Time, this is only an artefact of the belief of the people to who he is known as the Grim Reaper. To these people death is generally taken more pragmatically, the sages who know of such things in these areas see the Grim Reaper as belonging to the Sphere of Time. (Original Immortals set, DM's Guide, page 41)
Gylgarid: Initiate(5)/Temporal?, Sphere of Matter/Entropy?. Gylgarid is an immortal who has strong ties to minotaurs, though is also worshiped by many northern tribesmen (X13, 3) which assumedly means Heldanners or Vestlanders. This then implies that Gylgarid was either a minotaur, an immortal who created minotaurs or a mortal who killed lots of minotaurs. A minotaurs head is the symbol of Gylgarid (X13, 18) though this need not mean he is a minotaur he could be like Ruaidhri Hawkbane and have taken the likeness of the animal he killed as his symbol. Though it also stated that the minotaur is the likeness of Gylgarid (X13, 27), though it seems odd how "many" northern tribesmen would worship a minotaur. The final piece of evidence is that Gylgarid is described as a chaotic barbarian god of uncertain origin (X13, 17) whose followers even seems to be feared by local people (X13, 17).
I would contend that Gylgarid is a mortal who killed a lot of minotaurs for a very personal reason (much like Ruaidhri Hawkbane) and the whole thing sent him a bit mad, nevertheless he was pretty proud of himself. He somehow attained immortality in the Sphere of Entropy (the rules for attaining immortality in this sphere have always seemed a little sketchy) and the whole experience of attaining immortality sent him over the edge. He took the minotaur as his symbol and seems to prefer his temples adorned with the figure and even acting as guardians. I think that Gylgarid was someone from this region of Mystara which accounts for it being his base of operation, though I think that he is not in fact worshipped by "many" northern tribesmen but perhaps only by a few similarly mad individuals, which is why his worship is not prominent in the northern regions, though he may become so. If this were all so then Gylgarid would probably not be a powerful immortal which is why I place him at Initiate(5) level, though X13 seems to imply the Gylgarid has some power so perhaps some Entropic immortal is working behind the scenes pulling his strings - though this is only conjecture.
Though all the above may not be so and the next most likely scenario is the Gylgarid is a minotaur who achieved immortality in the Sphere of Matter, and would probably therefore be a Temporal. Though he has some unsavoury followers.
Hymir: Celestial(16), Sphere of Time. Hymir was a chronomancer from Sundsvall about 200 BC. He was the Archchancellor of the University of Sundsvall. Not having any children he attained immortality by increasing the size of the university, particularly the chronomancy department, and also ensuring its survival through time, his final trial being to perfectly rebuild the university after the sinking of Alphatia. Hymir was always fond of his drink and it is no wonder that the chronomancy department is known for the best ale in Alphatia, it should also be noted that a significantly high number of the archchancellors subsequent to Hymir have also come from the chronomancy department - most of them of course were also fond of their drink. Since reaching immortality Hymir has remained interested in the affairs of Alphatia as a whole though he also found many allies in the northmen pantheon who are also fond of their drink. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Idraote: Celestial(16), Sphere of Energy. Idraote was a mage on the planet of Alphatia who attained immortality a long time before its destruction. He followed his people to Mystara although he is not overly prominent in the affairs of Alphatia. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Idris: Temporal(8), Sphere of Entropy. Although the symbol of Idris is a black dragon (X11, 17) I do not believe that she was a dragon in real life (a thought which will be expanded upon in a later post). Instead Idris was a mage who was a member of the craft of Dracology. [Note: here I am assuming that the craft existed before the Great School of Magic in Glantri, and that this craft and the others were collected under the banner of the School once it had opened]. She attained high mastery in this craft and was fond of turning herself into a black dragon and harassing people in the region. Some of the people who witnessed her savagery began to worship the dragon, some perhaps out of fear but some did so because they were impressed at the power that she possessed. Idris went on to attain immortality in the sphere of Entropy who were impressed by her blind brutality and thought that they may one day have a need for her. This information will undoubtedly change soon.
Ilmarinen: Celestial(15), Sphere of Matter. Ilmarinen was a Kogolor dwarf craftsman by the name of Tordur. He was sponsored to immortality in 200 BC by Garl Glittergold who was very impressed with his sense of art and his practicality. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Korrigans, The: Empyreal(21), Sphere of Energy. The Korrigans are a group of nine elves who lived in the forest areas in the area which would become Wendar, although they lived there a long time ago even before the coming of the Antalians. They all adventured together in mortal life but parted when each chose to attain immortality. Unbeknownst to them they were all being sponsored by the same immortal who enjoyed the precedence that he was setting. The nine elves all achieved immortality within a few centuries of each other and then resumed their friendship. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter) This information will undoubtedly change soon.
Light: Hierarch, Sphere of Energy. This immortal is merely Ixion under a different name. (Original Immortals set, DM's Guide, page 41)
Mother Earth: Hierarch, Sphere of Matter. This immortal is merely Terra under a different name. (Original Immortals set, DM's Guide, page 41)
Old Nick: Hierarch, Sphere of Entropy. This immortal is merely Thanatos under a different name. (Original Immortals set, DM's Guide, page 41)
News: Hierarch(36), Sphere of Thought. News is an old immortal who lived in the city of Blackmoor. She acted to inform the citizens of Blackmoor against their actions by collecting any information she could find and then talking to the citizens of Blackmoor. Although she was an excellent orator she had not swayed enough people by her knowledge by the time of the Great Rain of Fire. Although she died in the ensuing cataclysm the immortal Diulanna was so impressed by her actions that she sponsored News to immortality. (Original Immortals set, DM's Guide, page 41)
Night: Hierarch, Sphere of Entropy. This immortal is merely Nyx under a different name. (Original Immortals set, DM's Guide, page 41)
Ninsun: Empyreal(19), Sphere of Energy. Ninsun was a winged minotaur in the ancient land of Nimmur. She attained immortality in the sphere of Energy with Ixion as her sponsor. Since attaining immortality she has shown little interest in the affairs of enduks. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Ouranos: Hierarch(36), Sphere of Matter. Ouranos is a very old being and unbeknownst to the other immortals is actually no longer an immortal. Millions of years ago a being existed which knew itself as Ssaa Kraat. Ssaa Kraat attained immortality under a long gone immortal of the sphere of Matter. Ssaa Kraat rose through the ranks of immortality and became the Reigning Hierarch in the sphere of Matter. Not happy with this position Ssaa Kraat reincarnated itself as a mortal in the hopes of learning more about the universe. The new mortal identity which also existed long before humankind knew itself as Ouranos. Ouranos achieved immortality in the sphere of Matter and also became the Reigning Hierarch in the sphere of Matter. So doing Ouranos learnt of the previous existence of Ssaa Kraat. Displeased with the outcome Ouranos decided to do as Ssaa Kraat had done - to become mortal again. So doing Ouranos passed beyond the barrier and went to exist with the Old Ones. The being known as Ouranos is known to the present group of immortals as an ancient being which, as far as they know, has gone to walk amongst the planes. Although there are a number of cultures on Mystara and throughout the Multiverse who know of Ssaa Kraat and the ultimate knowledge which is attained by that being. Ssaa Kraat is known by many names including Zagrat, Saccrat and Sakkrat. Ouranos is not tied in with any of these legends. (Original Immortals set, DM's Guide, page 41; Master DM's book Artifacts chapter and The Highest Science by Gareth Roberts)
Pyro: Hierarch, Sphere of Energy. This immortal is merely Ixion under a different name. (Original Immortals set, DM's Guide, page 41)
Saturnius: Celestial(15), Sphere of Energy. Coloc was a boy born in 3746 BC in a tribe which has previously been assimilated by the Oltecs. He grew up resenting the form of imprisonment which the Oltecs practiced. One day he left the tribe and adventured around Mystara. He attained immortality in the sphere of Energy and since then has acted as a patron of freedom. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Simurgh: Celestial(15), Sphere of Time. Simurgh was a member of a race of roc like birds which were the ancestors of present day rocs. Simurgh attained immortality in the sphere of Time and as part of the trial he averted the death of his people from water, ice and fire. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Skuld: Celestial(14), Sphere of Entropy. Skuld was a Hill Giant who hated his existence and hated his parents for bringing him into it. As a result he killed them, the not happy with this he decided to kill his relatives. Although the science of genealogy as practiced by Hill Giants is not very precise so Skuld killed any Hill Giant he came across. So impressed with his record of killing Skuld was sponsored to the sphere of Entropy. He now acts against all giants although he still particularly hates Hill Giants. (Original Immortals set, DM's Guide, page 41, and Master DM's book Artifacts chapter)
Solarios: Hierarch, Sphere of Energy. This immortal is merely Ixion under a different name. (Original Immortals set, DM's Guide, page 41)
EVEN MORE IMMORTALS
These immortals don't have anything written about them.
Olean (GAZ 9, page 44) this is a junior immortal, probably Initiate, who is trapped within the Carven Oak.
Creator of the Carven Oak (GAZ 9, page 44).
Creator of the Faenare, sphere of Thought (PC2, page 6).
Creator of gremlins, sphere of Entropy (PC2, page 13).
Creator of harpies, sphere of Entropy (PC2, page 18).
Any of the above four immortals
could easily be one of the already known immortals.