How does Possession interact with a unit that has Gear equipped, and what happens to both cards when the possessed unit dies or is bounced to hand?

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When you play Possession you take control of the targeted enemy unit and then recall that unit to your base. Any Gear that had been attached to that unit does not go to your base with the unit; it remains at the battlefield and will be Recalled to its controller’s base during the next Cleanup. If the possessed unit is later killed, it is placed in its owner’s Trash; if it was merely bounced to its owner's hand, the unit goes to hand while the Gear is still recalled to its controller’s base at cleanup. [452] [323] [808]

Resolve Possession: you gain control of the unit, then it is recalled to your base (per the spell text). Attached Gear stays behind at the battlefield and will be Recalled to that Gear’s controller’s base during the next Cleanup (see the example in [452]). If the possessed unit is later killed while you control it, the killed unit is placed in its owner’s Trash (killed permanents go to their owners’ Trash) and any attached Gear—having already been left at the battlefield—will have been Recalled to its controller’s base during Cleanup. If the unit is bounced to its owner’s hand, the unit goes to hand and the attached Gear is still Recalled to its controller’s base during the next Cleanup. Deathknell on the unit only triggers if the unit is actually killed and sent to the Trash; a recall in place of a kill prevents a Deathknell trigger ([808]). [452] [808] [323] [359]

Cast Possession targeting an enemy unit at a battlefield. The card instructs: "Take control of it and recall it. (Send it to your base. This isn't a move.)" Resolve the control change first, then apply the recall instruction (this is what the spell does). When the unit is Recalled off the battlefield to your base, any Gear that had been attached to it remains at the battlefield (the Gear was "present at that battlefield") and is therefore subject to the Recall rule for Gear: that Gear will be Recalled to its controller's base during the next Cleanup [452]. If the possessed unit is later killed while you control it, the killed unit is placed in its owner's Trash (killing sends units to their owners' Trash) and Deathknell triggers only if the unit was actually killed and sent to the Trash; if the unit was not sent to Trash because it was Recalled instead of killed, Deathknell will not occur [808] [323]. If the possessed unit is bounced to its owner's hand (returned to hand), the unit leaves the battlefield into hand; the attached Gear—left behind at the battlefield—will be Recalled to its controller's base during the next Cleanup [452]. If any other card or effect explicitly moves Gear or overrides these behaviors, follow that card text and the normal rules for partially-executable instructions and changed legality checks (see [359] for guidance on checks returning null when a targeted object changes location).

If an effect simultaneously kills the unit and would move it somewhere else, or if an effect would both Recall and Kill the unit, timing and replacement effects matter. If the unit is not sent to Trash because a recall replaced the kill, Deathknell will not trigger [808]. If a spell or effect that occurs while the unit is being targeted changes the unit's location mid-resolution (so that some follow-up instruction on a resolving effect can't be executed), apply the partial-execution and null-information rules in [359]. For example, if an interaction causes the unit to be bounced or recalled before some later instruction refers to it, those later checks may return null [359]. If the Gear was controlled by the player who gains control of the unit (i.e., the possessor somehow also controlled the Gear), the Gear’s controller is still the player who controls it; Recall sends Gear to its controller’s base, not necessarily the unit’s new controller. The ruling above assumes Gear remains under its original controller unless a card specifically changes Gear control. The provided Core Rules do not include a line explicitly stating that attached Gear 'drops off' when a unit is Recalled; the ruling above uses the example in [452] (Equipment present at a battlefield is Recalled when its host dies) as the best available support for treating attached Gear as remaining at the battlefield and being Recalled to its controller’s base during Cleanup.

Text of Possession: "[Action] (Play on your turn or in showdowns.) Choose an enemy unit at a battlefield. Take control of it and recall it. (Send it to your base. This isn't a move.)"

When Possession resolves you perform two things: you take control of the target unit, then it is Recalled to your base per the spell text. The Core Rules excerpt about Gear and Recall says that Gear that is present at a battlefield will be Recalled to its controller’s base during the next Cleanup; the example explicitly covers an Equipment that is attached to a unit at a battlefield and states that if the unit dies, the Equipment will be Recalled during the next Cleanup [452].

Applying that language to Possession: the attached Gear is "present at that battlefield" while it is attached. When the unit is Recalled to your base by Possession, the Gear does not travel with the unit (there is no rule text in the provided excerpt that moves Gear from a battlefield into a base with the unit). Therefore the Gear remains behind at the battlefield and is treated as Gear "present at a battlefield" and is Recalled to its controller’s base during the next Cleanup [452].

If the possessed unit is later killed while you control it, the killed unit is sent to its owner’s Trash (the rules indicate killed permanents go to their owners' Trash) and any Deathknell on that unit will trigger only if the unit is actually killed and sent to Trash. If instead the unit was not sent to Trash because the unit was Recalled in place of being killed, the Deathknell will not occur ([808] explains that a Deathknell does not occur if the permanent isn't sent to the Trash because its killed event was replaced with a recall). The provided cleanup rules remind you that Recall of unattached Gear happens during Cleanup, which is where the attached Gear that was left on the battlefield will end up being moved to its controller's base [323] [452].

If the unit is bounced to its owner’s hand (returned to hand), the unit leaves the battlefield to hand; the attached Gear—left behind at the battlefield—still meets the condition in [452] as Gear that is present at a battlefield and will be Recalled to its controller’s base during the next Cleanup. Also note that if some effect changes the unit’s location while other instructions or checks are on the chain, you must follow the partial-instruction and null-information rules in [359], which can affect whether subsequent instructions that reference that unit can be completed [359].

Because the provided Core Rules excerpt does not include an explicit rule that attached Gear moves with a unit to a base, the most defensible interpretation (and the one supported by the example in [452]) is: Possession moves only the unit to your base and any attached Gear stays behind and is Recalled to the Gear’s controller’s base at the next Cleanup. If a later effect kills the possessed unit (so it goes to Trash) Deathknell triggers normally; if instead a recall replaced the kill then Deathknell does not occur [808].