Path of Exile is a free to play action-RPG game similar in style to the Diablo franchise. A game focused on deep customization through using unique feeling characters. This is all offered for free, with in-game purchases available for cosmetic items and improved inventory storage.
Pro Expansive skill tree
While it may be difficult to take in at first the skill tree in Path of Exile is quite expansive allowing for many areas of the characters skills to be customized. The tree is so large there are numerous guides online on how to build specific optimized characters as well as an unofficial companion app for Android.
Pro Completely free to download and play
Just install steam and download the game. No purchase required and the only thing locked behind paywalls is cosmetic accessories and inventory slots of which you can waste money on cosmetics, but you won't fall behind if you don't want to spend money.