Sithis, Nessian Viper

{5}{g}: Monstrosity 3 (If this permanent isn't monstrous, put 3 +1/+1 counters on it and it becomes monstrous.)

When Sithis, Nessian Viper enters the battlefield or becomes monstrous, Snakes you control get +X/+X, where X is his power.
As long as Sithis is Monstrous, You may cast snakes as though they had flash and snakes you control have menace.