In other words, if you sacrifice Heart of Bogardan rather than pay its cumulative upkeep, X is equal to the amount of mana last spent to pay its cumulative upkeep.
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.)
When a player doesn't pay this enchantment's cumulative upkeep, this enchantment deals X damage to target player or planeswalker and each creature that player or that planeswalker's controller controls, where X is twice the number of age counters on this enchantment minus 2.
