Cheapest priced copy (The Lost Caverns of Ixalan): $3.99 at Card Kingdom, near mint, their price list as of .
Matzalantli, the Great Door // The Core 
Legendary Artifact // Legendary Land · Rare · The Lost Caverns of Ixalan · #387
{T}: Draw a card, then discard a card.
{4}, {T}: Transform Matzalantli. Activate only if there are four or more permanent types among cards in your graveyard. (Artifact, battle, creature, enchantment, land, and planeswalker are permanent types.)
(Transforms from Matzalantli.)
Fathomless descent — {T}: Add X mana of any one color, where X is the number of permanent cards in your graveyard.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
2 printings · $3.99 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan | 256 | Rare | $3.99 | $5.99 |
| The Lost Caverns of Ixalan | 387 | Rare | $4.99 | $6.99 |
4 rulings from Wizards of the Coast
- 10 November 2023Cards with the ability word "fathomless descent" have abilities that care how many permanent cards are in your graveyard.
- 10 November 2023The Core's activated ability is a mana ability. It doesn't use the stack and can't be responded to.
- 10 November 2023Transforming Matzalantli doesn't cause it to leave the battlefield. It also doesn't cause it to become untapped.
- 10 November 2023Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.