Rather than adding a portfolio element to Demogorgon's that doesn't match his others, I think it's better to just make Stodos a new immortal.
Stodos
by Ripvanwormer(Cryonax)
Master of the Icy Wastes
Level, Alignment, Sphere: 3 (Temporal - Croaking Demon), NE, Entropy [Stasis]
Symbol: Crystalline toad (ideally carved from magical unmelting ice)
Portfolio: Ice, cold, arctic wastes, multiversal heat death
Worshipped in: High in the Cruth Mountains, other cold "frostfell" regions
Appearance: Enormous, demonic toadlike biped
History: LIKELY THEORY: Stodos was a descendent of those frogfolk created by Saint Stephen in the early Blackmoor era. He was an accomplished technomage, but extremely introverted, even antisocial. Feeling out of place in the human-dominated academic world, he eagerly joined one of Blackmoor's moon missions in 3205 BC. Stodos grew to love the pristine chill of the moon and hate the Blackmoorian bureaucrats who attempted to bring him back once his shift was through. Working in secret, he created a bacterium that would kill humans but leave frogfolk unharmed, thus causing the failure of Blackmoor's first colonisation attempt. The other frogman in the crew he killed more crudely with a large, club like wrench. As far as the mission commanders at home knew, the deaths were completely unexplained, and they assumed no one survived. Unbeknownst to Stodos, this fulfilled the "create a unique magic item" requirement of the Path of the Paragon.
Stodos cannibalised as much of the technology in the moon base as he felt he needed and constructed a secret base some distance away, using mechs to help him transport the materials. Then he destroyed the remains of the base utterly enough to cover his tracks, thereby fulfilling the Sphere of Entropy's requirement that its questers destroy a lesser artifact of the Sphere of Energy - the base and its power, which he and other Blackmoor technomages had laboured on for decades. While he created heat and atmosphere in his new base for a time, he worked primarily on altering himself so that he could survive on the barren surface of the Moon without any artificial aid.
Over the next few years, Stodos destroyed several more colonisation attempts, working in secret to sabotage key components or creating new tailored bacteria that the colonists would not detect in time. Thus it was that he easily proved himself the greatest techno-wizard in far more than a thousand miles. He also expanded his base, creating a labyrinth of tunnels and experimental laboratories beneath each subsequent lunar base, transforming the terrain beneath the lunar surface to a surprising extent.
In the planes of Entropy, Hel noticed Stodos' feats and saw in him a kindred spirit who had nearly completed the requirements for Immortality entirely on his own. She granted him a vision of her home plane, Niflheim, and hinted at the power buried there. That was all that Stodos needed - he used his expanded labs to research the means to create an interplanar gate, and used it to visit Niflheim under his own power.
Stepping into the freezing fog and snow of Niflheim, he came face to face with Hel herself. She explained to him about Immortality and his real chances of achieving it: he needed only to train a group of apprentices, which he was welcome to do in her service rather than bothering himself with the wretched inferiors of his own world. Impressed with this offer, Stodos accepted.
As a mark of his new status, Hel transformed Stodos into a hezrou, a croaking, demonic creature of great power. In this new form Stodos taught magic and technology to six of Hel's fiendish servants, teaching them many of his secrets - though not all of them. When Hel deemed the last apprentice sufficiently accomplished, she rewarded with their teacher with true Immortality.
In the millennia since, Stodos has been up and down the immortal hierarchy. His greatest success was helping to destroy Blackmoor civilisation and its interplanetary colonies once and for all; after this was complete he became lost for a time, unsure what his next goal should be, and he was defeated a number of times in the immortal Olympic games.
Ultimately, Stodos decided that only when all the worlds were rendered as barren as Niflheim or the Moon would he be content. In this he was opposed most strongly by the immortals of the Sphere of Energy, and due to the efforts of Rathanos, Ixion, and others he hasn't yet made much headway since the last Ice Age. Still he builds his mortal following in the coldest places of the world, he recruits armies of icy beings on the elemental planes, and he creates endless new varieties of icy monsters in his home plane, called the Chiselled Estate. He is patient and knows one day victory will be his.
Stodos allies himself with the gods of Ragnarok: Hel, Orcus, Demogorgon, Loki, Alphaks, and Thanatos. However, he has his own agenda and works to ensure that the worlds end not with fire, but with ice. For this reason, Zugzul has taken violent exception to his means (if not his end), and has become one of his greatest foes.
ALTERNATE THEORY: Stodos is a far more ancient deity, a being of cold and chill during the Age of Serpents who brought many troglodytes and lizard folk to serve it in the arctic wastes later known as Nithia. Its people unsuccessful in the cold even when Stodos' magical aid, it had long dwindled and eventually sunk, with its mortal followers, into a sleep of many centuries. It awoke, finally, to the sound of strange engines. Seeing potential, it persuaded a scientist of Blackmoor to create for it a race of froglike servitors. When this race was largely wiped out by followers of the demon-god Zugzul, Stodos began concentrating all of its efforts into destroying Zugzul and all it stood for along with all fire, heat, and energy.
Personality: Cunning, creative, patient, hateful, sociopathic.
Patron: Hel
Allies: Orcus, Demogorgon, Loki, Alphaks, Thanatos
Enemies: Rathanos, Ixion, Zugzul, Odin, Frey, Freya, Djaea, Ka the Preserver
Classic D&D Stats:
Followers' Alignment: Chaotic
Favoured weapons: Club (allowed: club, dagger, natural weapons)
Clerics' spells & powers: Permanent Resist Cold
Avengers' skills & powers: Permanent Resist Cold
Domains: Air, Cold, Evil, Storm, Winter, Water
Preferred weapon: Club
Sources: M1