Characters can challenge each other, though in a neat twist, you can only Challenge characters who are Exerted
similarly, if I know they have multiple Ancient Grudges, Ill bring in Grafdiggers Cage
The old mantra, "Dies to Doom Blade" comes to mind
Cards like Sevinne's Reclamation and Faithless Looting are classic flashback examples, and it showcases a typical quirk: Alternative costs usually come with some conditional (in this case, be in your graveyard) that you have to fulfill if you want to cast the spell for its alternative cost
Plane - Alara Ketria When you planeswalk to Ketria and at the beginning of your upkeep, put your choice of a vigilance, menace, or trample counter on target creature you control
Lands don't cost any mana, but you can usually only play one a turn, and like other MTG card types, you can only play them at Sorcery speed