Nyxborn Unicorn

Mentor is a triggered ability. “Mentor” means “Whenever this creature attacks, put a +1/+1 counter on target attacking creature with power less than this creature’s power.”

If a creature has multiple instances of mentor, each triggers separately.

An ability that triggers whenever a creature mentors another creature triggers whenever a mentor ability whose source is the first creature and whose target is the second creature resolves.

Bestow represents a static ability that functions in any zone from which you could play the card it’s on. “Bestow [cost]” means “As you cast this spell, you may choose to cast it bestowed. If you do, you pay [cost] rather than its mana cost.” Casting a spell using its bestow ability follows the rules for paying alternative costs (see 601.2b and 601.2f–h).

As a spell cast bestowed is put onto the stack, it becomes an Aura enchantment and gains enchant creature. It is a bestowed Aura spell, and the permanent it becomes as it resolves will be a bestowed Aura. These effects last until the spell or the permanent it becomes ceases to be bestowed (see rules 702.103e–g). Because the spell is an Aura spell, its controller must choose a legal target for that spell as defined by its enchant creature ability and rule 601.2c. See also rule 303.4.

If a bestowed Aura spell is copied, the copy is also a bestowed Aura spell. Any rule that refers to a spell cast bestowed applies to the copy as well.

When casting a spell bestowed, only its characteristics as modified by the bestow ability are evaluated to determine if it can be cast.

As a bestowed Aura spell begins resolving, if its target is illegal, it ceases to be bestowed and the effect making it an Aura spell ends. It continues resolving as a creature spell. See rule 608.3b.

If a bestowed Aura becomes unattached, it ceases to be bestowed. If a bestowed Aura is attached to an illegal object or player, it becomes unattached and ceases to be bestowed. This is an exception to rule 704.5m.

If a bestowed Aura phases in unattached, it ceases to be bestowed. See rule 702.26, “Phasing.”