Official Rulings
If I have a 2-power creature with a Soulkeeper (AoA 83) attached, my opponent has a 6-power creature and a 5-power creature, and I play Opal Knight (AoA 260), do both of my opponent’s creatures die, or just the 6-power one?
Just the 6-power one. Since Soulkeeper’s effect resolves before the destroyed creatures leave play, it will try to tag the 6-power creature for destruction even though it has already been tagged.
Source: Rulebook
If I have a 2-power creature with a Soulkeeper (AoA 83) attached, my opponent has a 6-power creature and a 5-power creature, and I play Opal Knight (AoA 260), do both of my opponent’s creatures die, or just the 6-power one?
Just the 6-power one. Since Soulkeeper’s effect resolves before the destroyed creatures leave play, it will try to tag the 6-power creature for destruction even though it has already been tagged.
Source: Rulebook
Commentary
If there are multiple creatures that qualify as the most powerful, each is considered “most powerful”. If an ability requires the selection of a single most powerful creature, and multiple creatures are tied, the Active player chooses among the tied creatures.
Source: Most Powerful
FFG has clarified that nothing goes to the discard pile until all the Destroyed: abilities are done resolving. Destroyed: abilities are unusual in that they can actually interrupt each other and cause this kind of effect nesting. Creatures can be tagged for destruction by something like Soulkeeper and then other effects will interrupt before they actually hit the discard pile. The key is that everything tagged for destruction from these kinds of chain reactions will go to the discard pile simultaneously, and therefore in the order of the active player's choosing.
The cards are not considered to be "destroyed" until the cards are placed in the discard pile; a Tolas that has been upgraded with a Soulkeeper that is being destroyed won't ever earn an Æmber because Tolas and any creature Soulkeeper destroys will both be destroyed at the same time.
Source:
Edit this card's data.