I finally get layers and time stamps, but dependencies still elude me. Can you give a brief description of those?
The first thing you need to know to understand dependencies is that dependencies only ever happen within a single layer. Even if all of the effects involved in the dependency also have effects in other layers, the dependency only matters for sorting out their order in the layer in which the dependency exists. Their effects in those other layers are sorted out separately using the usual timestamp rules.
Now that that caveat is out of the way, let’s talk about dependencies themselves: A dependency occurs when we need to resolve effects in a given layer outside of their timestamp order in order for a more intuitive result to happen. Specifically, one effect depends on another effect if neither effect or both effects are characteristic-defining abilities and the second effect would change the text or the existence of the first effect, what it applies to, or what it does to any of the things it applies to.
So what does this mean? Let’s look at the criteria with some examples:
“Change the text or the existence of the first effect”
(I can’t think of an example for the text-changing part, and neither could a couple of other rules experts that I asked. It’s possible that this specific rule is a relic of a past revision to the layer system or is there just to prevent possible future shenanigans.)
Causing another ability to not exist can be shown by the classic example of Blood Moon and Urborg, Tomb of Yawgmoth: Applying Blood Moon’s effect to Urborg removes all of Urborg’s abilities, which causes its “All lands are also swamps” ability to no longer exist. This means that Urborg’s ability depends on Blood Moon’s ability and so Blood Moon’s ability has to be applied first.
“What the first effect applies to”
This criterion is met when the second effect either expands or narrows the set of affected objects. Dralnu’s Crusade and Conspiracy provide a good example here. No matter which creature type is chosen for Conspiracy, Dralnu’s Crusade’s ability will depend on Conspiracy’s ability. Either Conspiracy is making everything into a goblin and expanding the set that Dralnu’s Crusade applies to, or it’s making nothing a goblin and making Dralnu’s Crusade not apply to anything at all.
“What the first effect does to any of the things it applies to”
This mostly matters for control-changing effects. The easiest example I can present is Steal Enchantment enchanting Control Magic. Control Magic’s effect depends on Steal Enchantment’s effect because applying Steal Enchantment’s effect first changes who gains control of the creature enchanted by Control Magic.
So that’s a brief(?) rundown of what dependency is and how it works. If anyone has any further questions, please send them to my inbox. Thanks! :D