I'm currently running a DOTMM campaign with four level 7 adventurers, however one of them will be leaving after only 3 more sessions... Partially due to this circumstance and the monotony of dungeon crawling I want to artificially inject a short climax and boss fight into the campaign before this player leaves.
Campaign Background.
When the campaign initially started one of my players (Bob) thought it would be fun to be related to a quest giving NPC, Esvele Roznar, which I agreed to. Esvele was wary of the other players because one was Drow and and another was a Tiefling. She told Bob it was a bad idea to go with them, but he didn't listen.
Shortly afterwards Bob is killed and eaten by a group of giant spiders and the party fails to notify Esvele.
She now thinks the party is behind Bob's murder.
Where we are now.
Esvele has already sent an assassin to enact her vengeance on her behalf, however the party disposed of him after a brief scare.
Today I will run a session based on the fact that Esvele has lost contact with the assassin. She will place large bounties using her family's wealth and begin summoning invisible stalkers and yugoloths to kill them wherever they may be (streets of Waterdeep).
When the player's realize who is behind the targeted attacks I fully expect them to search for Esvele to try and stop her, which should take about 3 sessions.
Boss Battle
I either want Esvele herself to fight the group as a vengeful sorceress or wizard or to sacrifice herself for a summoning ritual of a demon or devil to enact her vengeance on the players.
I'm looking for advice on making this final boss encounter deadly but fun.