The Event Horizon (イベントホライズン, Ibento Horaizun) is a new Zone used for Timewarp Summoning, being much larger than a normal Zone (being as wide as the Monster Card Zones and Spell & Trap Card Zones) and located behind the Spell & Trap Card Zones. Monsters cannot simply be placed in the Event Horizon on their own: They require the use of a Spell, Trap, or monster effect, unless said monster effect is its own. When a monster is placed in the Event Horizon, any changes to its Level are removed, returning it to its original Level. During each Standby Phase that a monster is in the Event Horizon, that monster gains 1 Eon Counter. When a monster has enough Eon Counters, the player can remove that monster from the Event Horizon and use it for a Timewarp Summon.
Only monsters with Levels can be placed in the Event Horizon, so Xyz Monsters, Composition Monsters, Spectrum Monsters, and Hybrid Monsters with a Rank, Quality, or Wavelength cannot be placed in this Zone unless their Rank/Quality/Wavelength is being treated as a Level.
Keeping with the width of the Zone and the five-card limit in the other rows, the Event Horizon can only hold up to 5 monsters at a time. Despite being a clearly marked Zone, the Event Horizon is not actually considered part of the field, much like the Graveyard, so effects that effect cards on the field cannot affect cards in the Event Horizon: The effects must specifically target cards in the Event Horizon.