I'm considering either have them stumble into the war between Gith and mindflayers or finding the Corpse of a Titan.

Trying to figure out what some interesting encounters would be for the party that consist of a Hexblade Warlock, Horizon Walker Ranger, Oath of the watchers Paladin and a Rune Knight Fighter

Planning on having a large dungeon dive that vampires are going to watch the party do in mist form for entertainment. Going to make it the most stereotypical dungeon imaginable. Open to ideas to add

My party
1. Assassin rogue
2. Dreams druid
3. Aberrant Mind Sorcerer
4. Wild magic Barbarian
5. Dance Bard

What is missing from the party... usually a healer