Cliché Storm: Every standard high fantasy trope you can name, most played straight. Kay has said he wrote the books as a thrown gauntlet to those who he felt were bastardizing Tolkien's work by misusing the tropes.
Rakoth Maugrim, the Unraveler, is a deity from outside of Fionavar who loathes the world he had no part in making. Long ago, Maugrim arrived in Fionavar and began corrupting what he could with the intention of annihilating the world entirely. After freeing himself, Maugrim captures the heroine Jennifer and, vowing to take everything from her, rapes her, while taking the forms of her father and lovers to destroy all happiness she may have. Aware his rape has gotten her pregnant with his son, thus binding him to Fionavar, Maugrim gives Jennifer to a servant of his to rape and torture for a night as long as he kills her at the end of it. When Jennifer is saved, Maugrim later inflicts a brutal winter on Fionavar, killing many by freezing and starvation, while using his forces to ravage the land. When he meets his son Darien, Maugrim is gleeful about a chance to murder him, vowing to not only to kill him but to unmake him utterly as to regain his invulnerability.
Metran, First Mage to the High King of Brennin, at first seems to be a harmless, senile old man. Revealed as a servant of the aforementioned Rakoth Maugrim, assisting his master in returning from imprisonment, Metran arranges the massacre of heroine Jennifer's soldiers to deliver her to Maugrim to be raped and murdered. In the second book of the trilogy, Metran obtains the Cauldron of Kath Meigol, powering it by draining the lives of hundreds of minions, resurrecting them and killing them again to create a killing frost over the lands that dooms many people to die via cold and starvation. Another land suffers a killing rain from Metran's powers, poisoning and killing almost everyone within its borders. When finally killed, Metran is attempting to shift the death rain over to the High Kingdom to wipe it off the map, driven only by his greed, hunger for power and hatred towards Fionavar.
So many, let's start with the top one. In the third book Lancelot battles one of the oldest creatures in the world for a whole night and never wavers during the battle! Surely he couldn't win but when he somehow wields the beast's own hammer he finally kills it! God damn it's hard to top that!
Diarmuid comes close near the end.
Coll commanding his ship in a very violent storm.
Gereint sending his spirit out to help forge a link between Paul, the earth, and Liranan god of the sea at a critical moment.
Many, both for readers and in The 'Verse. (Characters will often weep when it would be the normal human response, unlike many works of the genre.) The Go Mad from the Revelation aversion is one notable example; learning the reason Paul wants to die is another.
The death of Kevin Laine
The scene that reveals what happened to Paul Schafer's fiancee, Rachel: they were driving home when she told him she was in love with someone else. Then their car crashed. She died, and he blames himself, feeling that he subconsciously rolled the car as an act of revenge.
Rakoth raping and trying to break Jennifer Lowe as well as the fate of her being Gwen and Arthur and Lancelot being her great loves
Dave Martinjuk learns that he has to leave Fionavar despite feeling like he belongs there. However he's learned a lot about himself and finds the courage to ask Kim out in the end.