I was thinking about the real final encounter in ToB, against Amelyssan the Blackhearted who is very near from major godhood.

I think the major theme could be murder : death effect, cold damage, magic damage, fear effect with appropriate pressure on saves vs death.

I was thinking it could be Amelyssan herself spawning blurred slayers(damage dealers), death tyrants(some kind of caster/debuffer/tank).

She could be allied with a very ancient dracolich entity with a unique name (dracolich but a lot stronger) which spawns skeleton grandlords and cairn wraiths (banshee avatar, fearful death dealing undead creature).

Each of the three seals to cut off Melissan's power would spawn a single fallen solar (beefed up a lot and annoying creature). First seal would have no effet beside releasing the fallen solar and saying that Melissan's power has been cut off. Second seal would reduce the speed at which she summons helpers and her control on her immensely powerful gated ally (would reduce dragon's own helper spawning speed as well as reducing some of his resistances). Last seal would break Melissan's invulnerability (she would be invulnerable at the beginning of the fight).

Incidentally, hesperus could grant immunity do death effect.

Just for the sake of discussion

Thanks, HTRT, all nice ideas!

I'd hesitate adding a dracolich entity seeing as it could be potential content for some of the quests. Nonetheless it's looks like a nice setup. Although I am a bit reluctant about the "hordes of monsters" scenario, realistically it's one of the few practical ways to make an encounter fun at that stage of the game.

If you think you can find some spare time whenever to write it up in greater detail (monsters types and stats, flow of the combat, stuff like that) we can code it up. Otherwise it would be postponed until an unspecified later date when I find more free time on my hands for some IA creativity.

I think this is a great idea - keep in mind that at this point the parties magical defenses are quite powerful and difficult to dispel (cleric level 39 death ward, saves vs death typically -4 or better). I personally don't enjoy mechanics where it is save or die (looking at you CC: 3x disintegrate after greater doom and Wail of Dracolich), but if done well it could be OK.

An immediately implementable option is to just replace the summons that Melissan uses with the IA improved ones - some elite fire giants, grave liches, greater bone golems, web golems, vampire lords, etc. could be challenging as an interim step.