What is this 18-word seed of random letters? I have a seed from a few years ago that looks like this:
this is not the actual seed, but it's random letters just like this:
nasd oasi lksn oifd nwee ckvj osdi pwer asem
pasd werw nase nwqe noas pmrr nolv psme ksdw

How can I recover the wallet?
It's an armory backup seed. See https://www.bitcoinarmory.com/. It's a wallet that requires bitcoin core full node so you'll need that as well.