Evelynn, Agony's Embrace

Evelynn, Agony's Embrace enters the battlefield
tapped.
For every two creatures in the graveyard
Evelynn, Agony's Embrace's mana cost goes up
by {1}.
When Evelynn, Agony's Embrace enters the
battlefield tap target creature.
{t}{x}: Destroy all tapped creatures, where X is
Evelynn, Agony's Embrace converted mana cost.