(Which we know explicitly from Halla's own opnion is kinda so-so, BTW. I think any runes on Sagaseeker should be the best we can manage and also say something important about the weapon.)
What Crowfeeder's runes do is ridiculously good mechanically and means our weapon will basically never break, which is excellent thematically as well. It's absolutely fabulous and I'm not very inclined to give up on it for something that might very well not do that specific thing. It's not the most poetic rune-phrase, but messing with what we know works in an attempt to be fancy is a good way to wind up with something that doesn't work.
Honestly I feel like killing a cat for this is kind of mean? Like it just feels vaguely unpleasant to me, why does a cat have to die?
I mean, I sort of assumed we just keep the bones of anything that dies around the place. Cats included. Acquiring such bones from our neighbors would likely also not be particularly difficult. Or hunting a lynx, which would surely count. Like, I don't see a reason using cat bones means we need to kill an actual domestic cat.
We should just use the Shadow Bear. Which I know, we've for some reason decided is only for endurance or stealth related effects, but... I seriously think you're overthinking it. The shadow bear a big powerful predator; the aspect of that which is most relevant to Sagaseeker and suits his whims of the moment will be what IF goes with when deciding on effects. And Realised weapons continually absorb stuff which strengthens them, so it's not like we can't try again.
I'm actually not super against using the Shadow-Bear for this for any reasons related to the shadow-bear (I was against using a predator
not known for endurance on armor...using one known for endurance on a weapon is potentially fine), I'm mostly pretty sure that
several other people would object strongly to this as there are Other Plans for the very limited amount of Shadow-Bear materials.
Ravens or crows would also work. I think a bird of prey in general would be a good idea.
Ravens and crows are scavengers, not birds of prey for the most part, and I think we do want a predator rather than a scavenger. A bird of prey, on the other hand, is fairly viable, but I'm not sure it's any better than a cat.
We can add runes to the wood without a full reforge, right?
The issue with this is that it would then take another action to add said runes, and we are not exactly overflowing with spare actions.
That's not exactly a dealbreaker, and leaving the haft un-runed is very possible, but it means we're probably not runing it up until we replace it with a magical material or something like that or come up with a really excellent rune phrase, or both. Like, this is a viable option, but it's one where we shouldn't expect those runes to get added any time soon. Eventually, yes, but not soon.