The veil tears open and the dark fills up with little wandering flames — that's you, and everyone else online. You're a will-o'-the-wisp, and you stay alive by eating the light other wisps leave behind.
Eat enough and you grow: a bigger flame, longer reach, and new weapons to unlock — the biomass blade, spore cannon, mitosis scythe and a few nastier ones. Then go hunt the other players before they catch you, and slip through the portals into the haunted forest and the lava lands.
Move: ZQSD or the arrow keys. Your wisp glides with a bit of inertia.
Attack: left click toward the cursor. A switches between the weapons you unlock as you grow.
Shield: hold right click to raise a shield that blocks projectiles (it drains energy).
Dodge: Space for a quick dash (costs a little light).
Grow: absorb the light on the ground. The brighter you are, the higher you climb the leaderboard and the more weapons you unlock.
Portals: walk up and click a portal to change world (forest, lava). Enemies there are much tougher.