Gons.io 4+

Gons.io throws you into a map where you must shoot to survive. Collect edges and shoot other players to gain more edges as you approach circular perfection.

Here's how it (basically) goes:

You materialize into a strange world as a humble triangle. You realize you are not constrained - but you can move! You find yourself to be in a small room, yet you feel the urge to explore. Look, a passage! You decide to enter. You travel around a corner and see something in the distance - what appears to be a spinning line segment. You approach it carefully. It doesn't seem harmful. You touch it and are immediately transformed into a square! It feels so good. You need to find more. You see what you think is another segment just obscured by a wall. You resume motion to check. Wait, that's not a segment - but another polygon! "Hello!" you exclaim, "Do you by any chance know where I could find some more of these line segments?" He doesn't answer and begins approaching you. Suddenly an edge is projected straight out of him. He is not giving you this - he is trying to kill you. You frantically dart sideways as the segment whizzes past your lower vertices. You scramble into rooms and down passages in an attempt to evade the assailant. This world is not what you thought it was. Those edges you want - everyone else wants them too - and they will do anything to get them. As you attempt to gather your thoughts, something amazing happens. You witness the miracle of spawn. A living, moving triangle - out of nowhere! After a brief state of awe, you remember where you are, and what you must do. "I'm so sorry," you mutter as an edge emerges from your polygonal body.