Its similar to the legend rule in the sense that both are automatic cleanups the game enforces, but they work differently: The legend rule only checks legendary permanents with the same name that a single player controls, and that player chooses what stays, while the world rule is table-wide, ignores names completely, and simply lets the newest world permanent overwrite the older one
Raise Dead, as discussed at length in the Wretched Confluence section, is also a really good effect to have on a modal card
With Infect such a prominent deck, Lavamancer's utility has only increased, which makes him a solid inclusion in many red-based strategies
Looking at the base of each deck: Lands
If this Spirit sticks around, its going to clear out an opposing graveyard or perhaps your own to power up its later abilities