This quote has stayed with me for some reason through numerous rereads. In EotW when Aginor says this to Mat was he just referring to Mordeth and the taint from the Shadar Logoth dagger or is it something more?

It's the most likely reason; Aginor was conscious even when imprisoned, so he would know about Mordeth. Aridhol was an enemy of the Shadow and also aided the Shadow unwittingly several times.

I thought he recognized Mat's soul, not the evil of the dagger. Altough the dagger makes equal sense.

No, it was the evil of Shadar Logoth. RJ commented on this--Aginor was aware and able to percieve the world, and thus knew of Shadar Logoth--a similar evil, yet an enemy.

old friend, old enemy; that seems to refer to Shadar Logoth.

old thing; that seems to refer to the dagger.

 

The only part that throws me off is "old friend". Mordeth was at odds with the shadow and his battle cry was "The victory of the Light is all" The evil of Aridhol is a very different kind of evil. Plus there was the army of shadowspawn including dreadlords that camped in the ruins slaughtered, can't think the dark one or forskaen were too stoked on that.

Mordeth tapped into some ancient things to gain his power. This is probably what's being referred to. An old friend, an old thing, and an old enemy all refer to the same thing, not three different. You see this sort of thing used at the end of love letters with less evil terms.