Citanul Centaurs

Shroud is a static ability. “Shroud” means “This permanent or player can’t be the target of spells or abilities.”

Multiple instances of shroud on the same permanent or player are redundant.

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].”

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.