Off the top of my head, in Standard and Modern, it prevents some or all of the use of the following cards: Every flashback card (in particular) Ancient Grudge Desperate Ravings Dread Return Dream Twist Faithless Looting Forbidden Alchemy Mystical Teachings Past in Flames Think Twice Unburial Rites Snapcaster Mage Narcomoeba Reanimate Exhume Every undying creature Every persist creature Gravecrawler Ichorid Ashen Ghoul Proclamation of Rebirth Polymorph Probably quite a few others 1) Drogskol Captain Hexproof is a very non-interactive creature mechanic, making it one of the more powerful mechanics to have been printed in recent years
It reminds me of Balan, Wandering Knight, but with a typal element should you choose to go that route
If you do, when the creature spell resolves and the creature enters the battlefield, it then makes a 1/1 token copy of itself
Entering as a 2/3 with Daybound that can provide target creature with +1/+0 and haste until the end of turn, this creature becomes strictly more powerful when it's night
A lot of cards on this list will be efficiently costed evasive creatures
2021-06-18 If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs