Can you trigger Elvenking on a ready hero, only to bring a silvan ally back into your hand?

No. If an ability's effect does not have the potential to change the board state, then you cannot pay the cost, or indeed trigger it in the first place. In this case, since you cannot ready the hero, you cannot pay the cost of returning a Silvan ally to hand.

RR Ability, Card Ability has the rules text, and this ruling explicitly addresses it as well:

In particular from there:

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If a card’s only effect is to ready a character, then you cannot play it on a ready character because it will not affect the board. So if Boromir is ready, you cannot raise your threat by 1 to ready him because it has no effect. Similarly, you cannot play Ever Vigilant to ready an ally who is ready because again it has no effect.

Cheers, Caleb

11 hours ago, sappidus said:

No. If an ability's effect does not have the potential to change the board state, then you cannot pay the cost, or indeed trigger it in the first place. In this case, since you cannot ready the hero, you cannot pay the cost of returning a Silvan ally to hand.

One has to wonder though if the "returning a Silvan ally to hand" of Elvenking and the Silvan events can be seen also as an effect and not only as a cost.

While often it's a thing you want to do, the card phrases it as a cost, and so it has to be treated as a cost.

On 3/3/2020 at 3:09 PM, sappidus said:

No. If an ability's effect does not have the potential to change the board state, then you cannot pay the cost, or indeed trigger it in the first place. In this case, since you cannot ready the hero, you cannot pay the cost of returning a Silvan ally to hand.

Interesting. I take it, then, you could do something like use Ceorl's action while he is readied, because readying him is just one of the potential effects that may occur (e.g. him being given to another player's control)?

17 minutes ago, EBerling said:

Interesting. I take it, then, you could do something like use Ceorl's action while he is readied, because readying him is just one of the potential effects that may occur (e.g. him being given to another player's control)?

This often involves parsing individual card text. In Ceorl's case, the " Then , you may give control…" would seem to prohibit triggering his Action when he is ready, as the effect after the "Then" cannot fire if what preceded it, i.e., the readying, did not occur. It would be different if Ceorl were worded like ally Sam, without a "then".

This whole principle is somewhat controversial—"potential" is never rigorously defined—and rulings around similar issues have been reversed in the past. But the above is how the current situation stands, I think.

Ceorl is a bad example. The way his ability is worded, you must ready him in order to give control. That's because the second sentence begins with "Then", meaning the first item must have resolved in full in order to do the second.

Lembas is a better example - as long as it is doing at least one of healing or readying you can use it.

On 3/5/2020 at 2:56 PM, rees263 said:

Lembas is a better example - as long as it is doing at least one of healing or readying you can use it.

Following this example (and making sure I understand this conversation) would it be safe to presume that you could also use the contract "The Grey Wanderer" and as long as you are able to resolve one of your two choices you are good? IE gaining two resources but already ready and at full health?

7 minutes ago, TheSpitfired said:

Following this example (and making sure I understand this conversation) would it be safe to presume that you could also use the contract "The Grey Wanderer" and as long as you are able to resolve one of your two choices you are good? IE gaining two resources but already ready and at full health?

Yes.

For reference:

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Action : Exhaust this card and raise your threat by 1 to (choose two): Ready your starting hero, add 2 resources to your starting hero's resource pool, or heal 3 damage from your starting hero.