Wait, that’s no Blood Moon.
Blood Sun is, clearly, a tribute to Blood Moon: both are enchantments with the same mana cost and both remove abilities from lands. There’s a catch, though: they don’t work exactly the same, so let’s see what makes Blood Sun different from Blood Moon.
Before getting started on this madness I’m going to explain what is a mana ability. A mana ability is an activated ability that would generate mana, does not target (e.g., no Deathrite Shaman), and is not a loyalty ability (those are the ones found on Planeswalker cards); or a triggered ability that triggers when activating another mana ability, would generate mana, and does not target. We could summarize all these (although it’s not an exact definition, but close enough for us for now) with “a mana ability is an ability that adds mana to a mana pool”.
Also, I should explain too that an effect that makes a land becoming of a specific basic land type without adding “in addition to its other types” removes all abilities from its rules text and previous land types, and adds the corresponding mana ability for each new basic land type (red mana, in Blood Moon’s case).
Now that that’s clear, let’s see some examples. We’ll start with Blood Moon, which is somewhat easier to understand.
With Blood Moon, a Temple Garden would only have the Mountain subtype, a mana ability that adds red mana, and no other abilities. We couldn’t pay 2 life to make it enter untapped, even if we wanted to, although it would enter untapped as the game would see it as a simple non-basic Mountain named Temple Garden entering the battlefield. What about a fetchland? Same. Urborg, Tomb of Yawgmoth? Ditto. A random non-basic land? That works, too. No non-basic land is safe: a Darksteel Citadel is still named Darksteel Citadel and is still an Artifact, Dryad Arbor is still a green 1/1 creature, but aside from that, the only thing they do is add red mana.
And now, let’s check what Blood Sun does, which is somewhat the same but without touching mana abilities:
A Temple Garden enters untapped and adds green or white mana, a Simic Growth Chamber does not return a land to your hand when it enters the battlefield and adds green and blue mana, a fetchland does not have any ability, Urborg, Tomb of Yawgmoth adds… black mana!
Even though Urborg’s does not have a mana ability printed on it, due to the way we apply continuous effects it’s still working. Specifically, effects that modify types always apply before effects that add or subtract abilities, so first we apply Urborg’s effect, making all lands Swamps (and giving them the ability to tap for black mana) and then they lose all abilities that are not mana abilities due to Blood Sun’s effect. Even if it’s a bit counterintuitive, in the end Urborg does not have the ability to turn other lands to Swamps, but they still are Swamps nonetheless.
With Blood Moon this does not happens because both effects apply on the same layer, and due to the dependency rules Blood Moon cancels Urborg’s effect before it has any chance to happen.
Back to Blood Sun: A City of Brass still adds any type of mana without problems, a Lotus Vale will not have any restriction when entering the battlefield (with the current text it has a replacement effect and not a triggered ability as it used to; not that it matters as Blood Sun affects both of them) and will give us a lot of mana, the Urza lands still check if you control the other two when resolving their mana ability, Unclaimed Territory still works as expected provided that you played it before Blood Sun resolved (if you play it after, you can’t name a creature type and hence the colored mana it produces cannot be spent in any ways. It would still add colorless mana…).
If you control other nine permanents (one of them being Arch of Orazca) and you cast Blood Sun, you won’t get the City’s Blessing when the Sun enters the battlefield because the land will not have the Ascend ability anymore: there’s simply no moment between you controlling 10 permanents and the land still having the ability when you could Ascend.
A rather interesting thing happens with Zoetic Cavern: if it’s face down as a 2/2 creature and you try to Morph it, you can’t: to pay the Morph’s cost you first have to turn face up the card, and when you do you only see an ability to add colorless mana (with Blood Sun out) or red mana (with Blood Moon out). Either way, you don’t have any Morph cost to pay, so you can’t turn it face up!
And last, but not least, Underworld Connections: if you annex the Connections after Blood Sun is played, the land will get the ability from it. Otherwise, the Sun will remove the ability! This happens because both effects apply on the same layer, but there’s no dependency: the only thing that matters in this case is the last one that entered the battlefield (and has a later timestamp): that’s the effect that will still apply.
To make things more interesting, doing the same with Blood Moon… does nothing! As I said earlier, Blood Moon removes abilities printed on the land, as well as abilities gained due to a copy effect, text-changing effect, or earlier type-changing effect, but not abilities gained due to ability-adding effects (like the one of Underworld Connections), because that layer comes after Blood Moon applies.
In short: Blood Sun leaves lands with their mana abilities intact and removes any other kind of ability, be that triggered abilities (like City of Brass), replacement effects (like the shocklands), activated abilities (like the fetchlands), or static abilities (like Reliquary Tower).
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