Kangaxx is immune to 1st through 9th level spells, but he does not have any native magic resistance per se. He has no resistance to fire, cold, acid, or electricity, so a few spells that create "real" fire or melee-inflicted damage such as Melf's Minute Meteors will hurt him. The Ring of Gaxx (which I presume he is wearing on his finger) gives him 10% magic resistance and immunity to poison and disease. The Ring of the Ram hits the target with a burst of pure magical energy. If you say it hits him, then it must be bypassing his immunity to spells but not his magic resistance.
On a sidenote, Kangaxx (who is a 27th level demi-lich) can be chunked by Turn Undead when used by a 34th level or higher Cleric, but the Demon Wraith (who is a 12th level demon) is not affected by Turn Undead.
Master Wraith
Well, I did as proposed and put my knowledge to the test. And here's what I found out.
The damage that the Ring of the Ram deals is not subject to magic resistance, the knockback effect of the Ring on the other hand is subject to magic resistance. At least that is, how it works in my game, which is unmodded, but not fully patched and not baldurdashed.
I suspect, that what you saw, was the feedback for the knockback being resisted. Perhaps you could check it again in your game?
Lark
The damage that the Ring of the Ram deals is not subject to magic resistance, the knockback effect of the Ring on the other hand is subject to magic resistance. At least that is, how it works in my game, which is unmodded, but not fully patched and not baldurdashed.
I suspect, that what you saw, was the feedback for the knockback being resisted. Perhaps you could check it again in your game?
Lark