For team games there is some ambiguity regarding mages' creatures that have received a special ability marker (Pet, Bloodreaper, Eternal Servant, Holy Avenger, Treebond, Veteran). The rules seem to not clearly answer the question what happens when the owning mage dies.
From the Battlegrounds rules:
Ability Markers:For the purpose of ability markers—such as Pet, Bloodreaper,
Holy Avenger, Undying Servant, or Treebond—the Mage that has that ability (or a
spawnpoint under their control) must be the one who casts the required object in order
to place that ability marker on it. For example, if the Warlock casts Darkfenne Bat, his
Beastmaster ally could not place his Pet token on it. The Beastmaster would have to Summon his
own animal, instead.
Loss of an Ally: If one of the Mages on your team is destroyed, the game isn’t over
yet! Remove his Mage card and destroy any objects attached to the Mage (such as
equipment or enchantments). All of the other objects he controlled remain in play,
and control of each object is transferred to another Mage on his team. Objects that are
attached to other objects, such as enchantments, stay attached to their current objects,
even though control of the card has changed.
Note that you cannot transfer control of some restricted objects. For example, if the
Warlock on a team dies, his creature Goran, Werewolf Pet is removed from play unless there is
another Warlock on that team who can take control of Goran.
It has been stated elsewhere (if someone can find the original source I'd be thankful), that the ability markers are not objects attached to these creatures but permanently change these creatures.
For the recent team tournament in Bonn I introduced the ruling that any creature with a mage's ability marker is removed from play when its owning mage dies. I would welcome an official ruling on this though. If a Bloodreaper or Treebonded tree should remain in play, how would these creatures function? They should retain there bonuses (Bloodthirsty +2, Innate Life +4, etc.) but have no effect besides that? Or should the creatures just lose those markers? What about Veteran markers?