As an additional cost to
castTo cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. A player may cast a spell if they have priority. See rule 601, “Casting Spells.”
To cast a card is to cast it as a spell. (CR 701.5) this spell,
blightTo “blight N” means to put N -1/-1 counters on a creature you control.
If a player is given the choice to blight but is unable to put N -1/-1 counters on a creature they control (usually because they control no creatures), they can’t choose to blight.
Some spells and abilities that instruct a player to blight refer to the “blighted creature.” This phrase refers to the object that the player who was instructed to blight chose to put -1/-1 counters on when blighting.
An ability that triggers whenever a player blights triggers after the process described in rule 701.68a is complete, regardless of what events actually occurred. (CR 701.68) X. X can't be greater than the greatest toughness among creatures you control. (Put X -1/-1
countersTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on a creature you control.)
Soul Immolation deals X damage to each opponent and each creature they control.