Cheapest priced copy (Tarkir: Dragonstorm Commander): $3.99 at Card Kingdom, near mint, their price list as of .
Ob Nixilis, the Fallen 
Legendary Creature — Demon · 3/3 · Mythic · Iconic Masters · #101
Landfall — Whenever a land you control enters, you may have target player lose 3 life. If you do, put three +1/+1 counters on Ob Nixilis.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
6 printings · $3.99 to $47.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Iconic Masters | 101 | Mythic | $4.99 | $11.99 |
| The List | IMA-101 | Mythic | $4.99 | — |
| Secret Lair Countdown | 15 | Mythic | $6.99 | $10.99 |
| Secret Lair Countdown | 42 | Mythic | — | $47.99 |
| Tarkir: Dragonstorm Commander | 192 | Mythic | $3.99 | — |
| Zendikar | 107 | Mythic | $5.99 | $24.99 |
3 rulings from Wizards of the Coast
- 8 November 2024A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.
- 8 November 2024Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).
- 8 November 2024A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.