Allegiance: Stormcast Eternals

If a friendly STORMCAST ETERNALS model is slain within 1" of an enemy unit, before removing that model from play, pick 1 enemy unit within 1" of that model and roll a number of dice equal to the Wounds characteristic of that model. Add 1 to the number of dice you roll if the slain model has the THUNDERSTRIKE keyword. For each 6, the target suffers 1 mortal wound at the end of that phase.

During deployment, instead of setting up a SCIONS OF THE STORM STORMCAST ETERNAL unit on the battlefield, you can place it to one side and say that it is set up in the Celestial Realm as a reserve unit. You can set up 1 unit in the Celestial Realm for each SCIONS OF THE STORM STORMCAST ETERNALS unit you have set up on the battlefield. At the end of your movement phase, you can set up 1 or more of the reserve units in the Celestial Realm on the battlefield, more than 9" from all enemy units.

You can pick 1 of the following subfactions for your army (core rules, 27.2.1). All STORMCAST ETERNALS units in your army gain the keyword of the subfaction you picked, and you can use the allegiance abilities for that subfaction. If a unit already has a different subfaction keyword on its warscroll, it cannot gain another one. This does not preclude you from including the unit in your army, but you cannot use the allegiance abilities for its subfaction.