Just got Wastes of Eriador, haven't played it yet. But I know that Predatory Wolves is going to be an issue for me when I do. So just to make it clear, can I or can I not choose to discard an ally when I don't have any allies?

Either... Or structured instructions mean that you have to resolve one of the options completely. If that happens to say "all allies" or something, that could miss, because 0 still counts as all. Predatory wolves however targets one specific ally, so unless you can discard one, you have to search for a warg.

You have to have allies to be able to sacrifice them... Also in the case of multiple allies tied for highest cost you can choose which one you discard. I found this insanely useful if you have multiple copies of a unique ally in your hand. Discard Arwen and just play her again next her.

RIP Beorn

Okay, thanks. There's been confusion in the past for me on cards like this, so thanks for clarifying.

haha yeah beorn ally gets decimated by this card. Pretty lolz too cause its a bear being taken down by wolves and not just a normal bear but a huge shapeshifting bear. Must be some pretty badass wolves to take him down.