There’s a weird thing that can happen with the gods of Theros when they’re entering the battlefield while you have 4 devotion. Even though the god is a creature spell on the stack and will be a creature AFTER it enters, the game doesn’t treat it as a creature entering the battlefield. The rules are a bit difficult to parse and understand, but we’ve received an official ruling to clear it up.
See the sister post to this one regarding triggered abilities when a god enters under 4 devotion here!
(WARNING: Technical details follow!)
When a permanent is entering the battlefield, you have to figure out which replacement effects apply to that event. To do that, you look at what the permanent would look like based on the current game state, accounting for continuous effects from the permanent’s own static abilities (e.g., Erebos’s “As long as your devotion to black is less than 5, Erebos isn’t a creature), but not from any other continuous effects that would affect it.
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply, check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see rule 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.
The hangup for many people is the line “check the characteristics of the permanent as it would exist on the battlefield,” because it seems to imply looking into the future for how it would look after it enters. But the reality is that you’re looking at the current game state BEFORE it enters, to see how it would look in that situation (i.e., while your devotion to black = 4). This is because replacement effects have to exist BEFORE an event actually occurs, so they can’t look into the future past the moment where they would be applying to see whether they apply during that event.
The current game state is “devotion to black = 4” because Erebos isn’t yet on the battlefield. So the replacement effect from Imposing Sovereign doesn’t treat Erebos as a creature entering the battlefield due to Erebos’s own static ability that generates a continuous effect (in Layer 4), so the Sovereign’s replacement effect can’t apply to it. That static ability only functions on the battlefield, but the rules for replacement effects take them into account as the permanent would be entering as well. The answer is the same if you have devotion to black of 0, 1, 2 or 3, as you might expect.
If you add a Whip of Erebos to the mix, so you have devotion to black = 6 at the moment Erebos is entering, then the Sovereign sees it as a creature that would be entering, so Erebos does enter tapped.
Another weird possibility is Thassa, God of the Sea entering while you control 4 Master Biomancers. Your devotion to blue is 4, Thassa will be a creature AFTER she enters, but she doesn’t enter with any counters or as a mutant because the Biomancers’ replacement effects don’t apply to Thassa with devotion = 4. The fact that your devotion will be 5 AFTER she enters doesn’t matter at all.
Yes, it’s really weird when dealing with devotion = 4, but this is the correct official answer.
Today’s Rules Tip written by Josh Stansfield