Echo

‘Echo’ was introduced in the Urza’s Block and it currently appears on a number of cards such as Bone Shredder, Basalt Gargoyle

702.27. Echo

• 702.27a. Echo is a triggered ability. “Echo [cost]” means “At the beginning of your upkeep, if this permanent came under your control since the beginning of your last upkeep, sacrifice it unless you pay [cost].”
• 702.27b. Urza block cards with the echo ability were printed without an echo cost. These cards have been given errata in the Oracle card reference; each one now has an echo cost equal to its mana cost.

Translation:

• 702.27a. Triggered at the beginning of the upkeep, if this permanent came under your control since the beginning of the last upkeep, you’ve got to pay this cost or sacrifice the creature.
• 702.27b. The original echo did not name a cost, it was just always the casting cost of the creature. Later it was modified to include a cost after the keyword.