Hosted creatures get +2/+0 and have haste. Host {1}{R}({1}{R}: Host target creature at this Realm. A Realm can host any number of creatures. Host only as a sorcery.)
Immersturm Battlefield is not legal in any format.
Rulings
2024-11-08
For many playtest cards, you'll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.
2024-11-08
Once a creature has been hosted at a Realm, that creature remains hosted until it or the Realm leave the battlefield.
2024-11-08
Playtest cards aren't legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone's on the same page about using them!).
2024-11-08
Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.
2024-11-08
You can target any creature with Immersturm Battlefield's host ability, not just creatures you control.