Cheapest priced copy (Duskmourn: House of Horror): $0.35 at Card Kingdom, near mint, their price list as of .
Erratic Apparition 
Creature — Spirit · 1/3 · Common · Duskmourn: House of Horror · #54
Flying, vigilance
Eerie — Whenever an enchantment you control enters and whenever you fully unlock a Room, this creature gets +1/+1 until end of turn.
Legal inCommanderBrawlStandardPioneerModernLegacyVintagePauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duskmourn: House of Horror | 54 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 20 September 2024An ability that triggers "whenever you fully unlock a Room" triggers when a door becomes unlocked and the other door of that Room is already unlocked, or when both doors of that Room become unlocked simultaneously.
- 20 September 2024If a permanent with an eerie ability enters at the same time as one or more enchantments, its ability will trigger for each of those enchantments.