If an opponent would mill one or more cards, they mill that many cards plus four instead
A handful of the best ways are Virtue of Loyalty, Chivalric Alliance, Forth Eorlingas!, Court of Embereth, and History of Benalia
Although it can't prevent combat damage that would be dealt to your planeswalkers, it can still prevent noncombat damage that your opponent would want to redirect from you to one of your planeswalkers
Incarnation Elementals The cycle of incarnation elementals is one of the strongest showcases of powerful enters triggers in Magic
attached to your leader, all of your characters will receive a point to their cost, making it a bit harder for decks like Rob Lucci to remove from the board
It was largely designed to make mulligans less painful in Standard and Limited