It's quite different from both in the way that the skills can have position mechanics for some like backstab (rogue class, least I am pretty sure they can backstab), or persuade/intimidate to sway conversant to your objective, but still xcom related in the way that each character has a turn where they can do a variety of multiple things in a turn. Diablo is only similar in that it has skills but an extremely different paced game. Positioning is important for spells, unless I am misremembering, because AOE in DnD doesn't care who's in the area, so you don't want to cast fireball when your party members are within the area.

Turn actions are broken down into their own categories like action / quick action (*later edit, bonus action when reading abilities/spells) which each has their own amount of (though usually a similar amount but after some levels some classes can hit/attack multiple times, this is needed for martial classes). The combat is turn related once started but you can often get characters into positions before starting combat (this may need stealth for some because once certain enemies see you they start combat). There is a bigger emphasis on role playing (conversation) choices in the game that can impact encounters, either with the current conversant or down the line. Certain actions like getting caught stealing will impact things too CRPGs are their own genre, have more in common with other Larian games (divinity original sin) or games like Pathfinder, and of course the older DnD games. The rules takes some knowledge and getting used to but not overly difficult, you can download a free edition of the DnD 5th editions rules which may help too(not 100% accurate but close enough). If you have friends I think you'll enjoy playing with them as you figure out your strategies from battle to conversations but it's a slower paced game. Just don't ignore things that can boost out of combat abilities to persuade (skill) or stealth that can give other opportunities while playing, though you can probably just play a murder everyone party if it works for you.

If you enjoy a quicker paced game though this isn't your game is all, it's slow and there can be a lot of time spent checking chests, talking and wandering. If you enjoy story and some tactical combat this is a good choice in my opinion.

Edit: forgot to add, party composition (classes) makes a big difference,you probably don't want a group of 4 of one class as each class has it's niche, but doesn't mean it's undoable may just be more difficult.

Later edit: I said quick action, but it's a actually bonus action, helpful for when reading the text, hopefully nobody was confused.