Exile Delayed Blast Fireball with any number of delay counters and an equal number of fire counters.
At the beginning of your upkeep, if Delayed Blast Fireball is exiled with any delay counters on it, remove a delay counter from Delayed Blast Fireball. If the last delay counter is removed in this way, Delayed Blast Fireball deals X damage to each damageable, where X is the number of fire counters on Delayed Blast Fireball, and then put Delayed Blast Fireball into your graveyard.
I changed it in some ways due to rule changes in the game; it is not quite the same as the original.