Darkrai EX is a meta defining S-Tier card that arrived with Space-Time Smackdown, and it created a completely new metagame that revolves about passive damage over time, high pressure and energy mobility. Darkrai ex is best left on the bench, shielded by a tank card (Koffing/Weezing), or by a high DPS card (Sneasel/Weavile ex).

Both of these cards can deal damage with single Psychic energy, which allows you to add energy to Darkrai ex. Darkrai ex's ability – Nightmare Aura – is what makes it so powerful, as it deals 20 damage every time you attach energy to Darkrai.

This, combined with the potential of having your opponent poisoned (by using Weezing), or bleeding damage (by using Weavile), is a devastating combination.

There is another way to run Darkrai in the infamous Darkrai ex / Magnezone / Druddigon deck, where Darkrai deals damage every turn, Druddigon is a tank that also deals damage when hit, and Magneton uses its ability to attach additional energy to itself in preparation for unleashing Magnezone.