Cumulative upkeep(At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
At the beginning of your end step, Magmatic Core deals X damage divided as you choose among any number of target creatures, where X is the number of age counters on it.
7/15/2006 The damage is divided when Magmatic Core’s ability triggers. Increasing or decreasing the number of age counters on Magmatic Core after the ability triggers but before it resolves has no effect on the amount of damage that’s dealt.
7/15/2006 If Magmatic Core has no age counters on it, the only number of targets the ability can have is zero.
7/15/2006 If Magmatic Core has at least one age counter on it, the number of targets must be at least one and at most X. Each target must be assigned at least 1 damage.
7/15/2006 Magmatic Core’s effect is mandatory. At the end of your turn, if the only creatures on the battlefield are yours, you must have Magmatic Core deal damage to at least one of them.