Did you play splix.io? This is actually is more advanced version of this io game. Here you can choose a custom flag which will represent your area. Also you have a capacity to use a special abilities for attacking and defending.
How to play Nafk.iO
Take a role of long worm inside a quite small territory that belongs to you. Then expand this territory and claim more for yourself by moving through the flag. Draw the border by moving your body. Do it as many times as you want and make a large flag, that was mentioned before, with some special abilities. Now you have 3 at your disposal, such as camouflage, speed up, and zoom out, which is not in fact so special, but anyway very useful in this cool io game.
Moving the body of your worm is arrow keys, as for special abilities use Q, W, and E. The abilities are shown in a little icon on your screen, so you would not confuse or forget them.