Zelthr'k, Eternal Nightmare

Devoid (This card has no color)
haste, indestructible, protection from creatures
Hex {1}{u}{b}{r} (If you cast this card for its Hex cost,
it's an enchantment aura curse spell with enchant
player. It becomes a creature again if it's not
attached to a player.)
Whenever player attacks enchanted player, or
attacks with Zelthr'k, untap all lands that player
controls. Then, they lose twice that much life.