A simple way to explain it is that any time something happens (spell, ability, attack, block, moving from one step of the turn to another), each player gets priority
A harder to cast mana leak, even with exile upside, isn't that good in Limited
That said, premium versions of Muldrotha easily surpass $100
Rigid This mechanic is very hard to use, requires extensive infrastructure to work, and/or is actively hostile when trying to mix with other mechanics
Magic: The GatheringFinal Fantasy represents their temporary assistance via Sagas, a form of Enchantment Creature that triggers various effects each turn, but only survives for a few turns
Dwell on the Past Dwell on the Past gets points for efficiency