Long story short, running an encounter for a group of 5 12th level adventurers; Tempest Cleric, Hexblade/Paladin, Skirmisher Rogue, Eldritch Knight, and Gunslinger. Long story short, plot hook with the Hexblade/Paladin's Knightly Order boss is corrupt, kill him.
He's already pretty much statted out, and the encounter is typically going to be fought with the party at full stats, and the Master and his mount at full health, inside the inner chamber of the fight; Copper Dragonborn, Barbarian Berserker 14/Fighter Cavalier 3/Warlock Hexblade Blade Pact 3, using the Cursebringer Invocation refluffed as a Spear, because the new one sucks, and he's riding a Half Green Dragon T-Rex. He'll be riding the mount before the fight.
With 5v2, and one of them only a low CR encounter, I'm unsure as to the balance of this. No extremely notable houserules relevant for this, except for all spells which restore life to dead people can only be cast as a Ritual. I'm guessing this is perhaps a CR13-14 encounter.
I want to throw a few Magic Items, and additional feats in there - maybe Mounted Combatant, Great Weapon Master, etc. One of the main ones is the Hexblade to which both are bound - a +3 counts-as Spear with the stats of a Greatsword, can do BPS damage, on a Crit Regains 1 previously spent Spell Slot up to 5th level, and 1/long rest, allowing bearer to cast 1 spell known by Caster at 5th level, plus a few little esoteric abilities.
Opinions of OP-ness, and whether I should throw the party a few extra levels in there, or if I should give some more feats/epic boons or magic items?
I'm a little unsure, as this is the first time I've really plotted a 5e encounter, everything else has been a bit by the book or a dungeon crawl nobody has really cared about, and doesn't matter if they win or die.
