I'm planning a one shot for two level 10 characters and I'd like a challenging boss fight for the end of it. The plan at the moment is that they will fight a corrupted Deva in the tomb of a Beholder God. The Deva will take the stats of the Deva from the Monster Manual, and the partially revived Beholder god will have a 1/4 chance of firing a random eye ray (again stats taken from Beholder from the Monster Manual) at one of the characters.
I've never run a boss fight like this before and have no experience with characters of level 10 or above (I've been DMing for a few years but usually for lower-level parties up to level 7-8). Any advice on this would be great, as I'd hate to have the fight be too easy or unreasonably difficult.
The characters are:
- level 10 Dwarf Cleric
- level 10 Gold Dragonborn Paladin