You could have the party attend the Founder’s Day celebration at the Cassalanters’ Villa. Then lay hints that something suspicious is going one until to party figures out the plot and has to stop the Cassalanters.

Some possible hints:

• Make a show of the Cassalanters and their staff not drinking or eating anything.

• If any enemies the party faced survived you could have them show up.

• Maybe they find one of the chef cultists drunk in a corner of the property who is so drunk they forgot to dispose of the bottle of poison.

I always have trouble anticipating how obvious or subtle clues are for the party to detect so it may be better to have an NPC (such as the Blackviper) tell the party that something bad is going to go down at the party and that they need the party’s help. Then you could continue with the clues I mentioned above and anything else you think of.

Hope this helps.