Planning on running an encounter at the end of a character arc and want to be sure I’m not pushing the party too hard. Party is all lvl 7 no multi class, 1 bard, 1 wizard, 1 paladin and 1 cleric.

Here’s the jist of my encounter: If you’ve played dark souls 3 it’s basically nameless king but a white dragon into a necrotic damage based paladin homebrew enemy. Scripted fight against young white dragon for 4 to 5 rounds, mostly doing environmental damage in anger, then the homebrew guy drops in, kills dragon, and the real fight starts. Dragon will do 1 breath attack for sure, others will depend on rolls. By then for a challenging fight I’d imagine the homebrew boss having about 150hp or so.

Any thoughts or suggestions? Is this too much for a lvl 7 party?