Io

Io does not untap during your untap phase. If it
remains tapped during your upkeep, Io deals 1
damage to you.

{4}: Untap Io. Use this ability only during your
upkeep.
{t}: Add {c}{c}{c} to your mana pool. Play these additions as interrupts.

"A moon of Jupiter, still rife with the energy of the Traveler."