Marvel Heroes. It was a diablo-clone with a lot of playable marvel characters - which you wither grinded a lot of shards to unlock - or bought them(they were expensive) -and then you grinded for skins (or bought them… Again expensive with random chance boxes).
It was cartoony, fun and easily still in the top 5 games by playtime on steam.
Rip marvel heroes, still miss ya, even if you were stupidly expensive for a f2p.