It reads:

Attach to a hero.
Action: Discard Miruvor to (choose two): ready attached hero, add 1 resource to attached hero's resource pool, attached hero gets +1 [Willpower] until the end of the round, or put Miruvor on the top of your deck.

If you attach to a hero of another player, and the other player chose "put Miruvor on the top of your deck". Where is it going to? To my deck or to the deck of the player that controls attached hero?

When you play an attachment on another player's hero, that player then controls that attachment, so when it says "your deck" it should be the deck of the player that controls the hero. I can imagine that's not always beneficial as they won't necessarily have Spirit resources.

I've just asked myself the same question...

In the rules, under "Control & Ownership" (p. 25) there is written:

A player “owns” his heroes and the cards that he has
chosen for the player deck he is playing. A player
“controls” all cards that he owns, unless another player
or the encounter deck takes control of the card through
a game effect. Any time a card leaves play, it reverts to
its owner’s hand, deck, or discard pile (as directed by
the effect forcing the card out of play). (emphasis mine)

When you play Miruvor on another player's hero, that player takes control of it, but you are still the owner of it. So, when it is discarded, it should go to your (=the owner's) discard pile, shouldn't it? What do you think?

that is correct. When miruvor is discarded, it leaves play. ownership of the card doesn't change

and when it is not discarded .. instead placed on top of your deck .. whose deck is meant? The owners deck or the controllers deck?

Controller's deck, as "your" refere to the controller, and it is golden rulling the ownership rules.

I'm still not convinced with this Miruvor when it is attached to another player's hero. I think it goes as intended and smooth when you play Miruvor on your own hero. You can then pick two of four given options. But when you play it on another player's hero, maybe that player has only three options after discarding Miruvor, from which he picks two? Or, he can still pick that fourth option when it would be useful in play (with permission from owner of Miruvor) and then that owner puts it on top of his deck?

When Miruvor is discarded, it is put on its owner's discard pile. If I am not the owner, I think it would not be possible to take it from another player's discard and put it on top of my deck. Moreover, the player who played it had resources to do it and I don't have to have Spirit hero to play it from my hand after drawing. If Miruvor is intended to circulate among players, maybe it should be simply neutral?

Well, it gives you the option, but if you don't have anyway to play it back, putting it on top of your deck is kind of dumb. But that's a problem with you, not with the card... Joking, I agree it's strange, but we were curious and we had an answer from Caleb, I think it is a "we didn't plan it but it's cool" design.

Controller's deck, as "your" refere to the controller, and it is golden rulling the ownership rules.

In the case of all of FFG's other LCG games, cards leaving play, as in the quote offered up by n Krokodiler, go to their owner's out of play area. You're correct, 'your' refers to the controller, but this destination change overwrites that. Wording in later LCGs has made that clearer (that it's not an exception to the golden rule), but it's been consistently ruled as such in every other LCG I've played. In the case of Miruvor, the controller would be able to choose any 2 of the 4 options, and if he chooses the 'top of your deck' it'll go to the top of the owner's deck, not the controller's.

I would be quite surprised if Caleb or Matt ruled otherwise. If they did, I'd love to see the specific wording.

Alogos is correct, and if fact this question was already asked of Caleb (full thread : link to reply shown below)

OFFICIAL REPLY:

Thanks Caleb!

Thanks, ricedwlit!

I would be quite surprised if Caleb or Matt ruled otherwise. If they did, I'd love to see the specific wording.

[...]

I would be quite surprised if Caleb or Matt ruled otherwise. If they did, I'd love to see the specific wording.

[...] and we had an answer from Caleb[...]

Yeah... let's just ask Caleb for the hundredth time.

I should make this my signature.

I hadn't yet seen that link, alogos. I appreciate it, and will look through it and get up to speed.

Incidentally, a search for "Miruvor" in that document yields no results.

That's stange, might be hiding in related question... ****, didn't made an update this week either... (the pdf version, and the French version I mean) ><.

EDIT: found and added.