Diep.io is a free, ad-supported download on the Play Store. The Google Play page says the game has in-app purchases, but I could not find evidence of these in-game.

Arras.io is a fan-made sequel to the 2D MassiveMultiplayerOnline game Diep.io. The gameplay is similar to the game it’s based on; at the start of the game, the player controls a simple Tank, and ...