I found this sentence on a visual novel, and it's the first time I see でも+あった at the end of a phrase, what does it mean?
This is the sentence:
Thank you in advance for your help
Conceptually speaking でもあった is what you get by trying to combine だった and も (as in "also").
だった is a contraction of であった and you have to use the uncontracted form in order to insert も after で.
So でもあった means "it also was".
(In the non-past tense, the same thing happens: "だ + も = でもある".)