Armageddon Clock is a rare Artifact card from Revised Edition (3ED), collector number 234. Current market price: $1.18, TCGplayer market prices via Scryfall.
At the beginning of your upkeep, put a doom counter on this artifact. At the beginning of your draw step, this artifact deals damage equal to the number of doom counters on it to each player. {4}: Remove a doom counter from this artifact. Any player may activate this ability but only during any upkeep step.