Vadrik, Astral Archmage 
Legendary Creature — Human Wizard
If it's neither day nor night, it becomes day as Vadrik enters.
Instant and sorcery spells you cast cost 
less to cast, where X is Vadrik's power.
Whenever day becomes night or night becomes day, put a +1/+1 counter on Vadrik.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL