Insidious Nightmares

When a creature enters the battlefield under an opponents control, Proliferate unless it's controller puts a -1/-1 counter on that creature.
When a card is exiled from anywhere, gain 1 life.
{2}{c}{c}: Each player sacrifices all non-Nightmare creatures tied for the lowest toughness, then creates an X/X Nightmare creature token where X is the greatest power among creatures they control.