Denial
by Stefan BeateI noted some ideas about the mechanics of a conversion here
Other than that, the Master is probably a modified cleric, with a special domain to reflect their special powers. I did not delve into the minutiae of the spell list to check which spells are missing from the 5e cleric list and need to be added via a domain. I donīt think that druid is an appropriate representation here, with their wildshape abilities.
The denial special power should probably be a feat. It would have a racial requisite, obviously. Iīm not sure if the level requisite should be kept, as feats in 5e come at a cost anyways. I donīt like the mechanics of this for 5e, with a d20 roll and adding stats to it. The power of the spell is no factor whatsoever for this ability, so this should be left out as well. The results given show that an average character will have no effect whatsoever 50% of the time, and a complete negation needs a very high roll and very high stats. For the most part, pretty useless, unless you happen to be a Master, in which case even average mental faculties and average rolls would grant full success - pretty imbalanced there, if you ask me. So, this roll might be better if keyed to a single mental stat. Iīd recommend Wisdom, but might give a choice of Wis or Int (or even Cha). Wisdom would keep it in line with the Master as divine caster. A table measuring success is not quite within the 5e spirit, but might be needed here. So, a Halfling with this feat and average mental stats would have minimal success on a roll of 11+ or more.
Maybe like this (note that a full success is possible for everyone here, this diverts from the original writeup):
roll: D20 + wis mod
up to 10 = no effect
11-15 = effect reduced by 1/4, slightly diverted
16-18 = effect reduced by 1/2, diverted
19 = effect reduced by 3/4, completely diverted
20+ = effect denied completely
nat 20 = effect reflected back on caster
I think relegating the reflection to chance is better than having this dependant on high rolls. If you want to keep the Masters advantage intact, you might want to add double the Wis Mod to the roll. This would not change the chance for a reflection, obviously.
EDIT: come to think of it, the Denial feat might be added as a 1st level power to the Master Domain. The double Wis mod should then probably be left out our added at a later point.