This comes from the movie. Strughold was the man in Tunisia that was introduced as possibly the boss-of-bosses of the Syndicate, played by the always-cool Armin Mueller-Stahl.

But did we ever hear about him again? Even in passing? Did I just miss it?

There had been foreshadowings of his character on the show before the movie (like the abandoned mine where Mulder and Scully found the huge "library" of human tissue samples - it had been owned by the "Strughold" company). But now nothing.

If I'm not mistaken the entire syndicate (except for CSM) were wiped out by the rebel aliens.

I guess we need to assume that Strughold was in that group that burned? Like I said, his name has never been mentioned once since the movie, even though he was introduced as a key player (perhaps THE key player).

I've wondered that myself. Here's the episode where "Strughold Mining Company" showed up:

http://www.thexfiles.com/episodes/season3/3x02.html

According to all these sites, Strughold only ever appeared in the movie. We don't know what's happened to him since.

http://www.turning-pages.com/xf101/x....htm#Strughold
http://www.turning-pages.com/xf101/xf101_bigpic.htm
http://www.geocities.com/Area51/Vault/4811/myth.html

We might assume that he either perished with the other conspirators in the hanger. But I think he's probably too smart for that. I think with the demise of the conspiracy, he just faded into the background. OR he might be directing the new, continuing conspiracy, with the super soldiers, et. al.

Or, the truth, that Chris Carter left this plot thread hanging so he could revisit it at his leisure, or not...fans be damned. See also, "continuity be damned."