Good final boss for a oneshot?

So, I'm making a oneshot for a small group of friends for dm practice and to test out online dnd.

My idea so far is that the party is level one or two being starting lower level adventurers, and are hired to go check on a lighthouse that has gone dark, causing some boats to crash.

The part will walk up to the lighthouse, it will be sunny, birds singing, the nearby town bustling. However, once they enter the house, its dim, its quiet. they look outside and its dark, cloudy, not nice.

Im going to make the floor fall out, or some magic trickery extend the house, and i was wondering if there are any experienced dms out there who have any ideas for a magical creature as a fun final boss that can do all these trickery spells.

Any ideas are welcome and appreciated!