(Dragonskull Key isn't a spell.)
At the beginning of your end step, if you didn't cast any spells this turn, Dragonskull Key deals 1 damage to you.
Dragonskull Key is indestructible. (Destroy effects and lethal damage don't destroy it.)
: Add 
to your mana pool. Dragonskull Key doesn't untap during your next untap step.