Trithereon the Summoner was not more popular among fandom. Sure he isn't a C-lister like Jascar or Joramy, but he never was quite good enough for "Core deity" status either. As far as I recall no one I've personally gamed with has ever made a priest or worshipper of the god of retribution. What's worse, back in 3rd edition Wizards folded Trithereon's retribution portfolio into St. Cuthbert, further burying this quite cool god.
Through my old World of Greyhawk Comic I used to explore the facets of many ignored gods like Trithereon. Undoubtedly his most interesting element is his notable animal companions. Trithereon is quite like the Beastmaster in this respect, a ranger-like vigilante bent on taking down evil with his animal cohorts serving useful roles.
Here's another. I'm sure few remember from the 1983 Guide that Trithereon also has his own "prison" demiplanes. The colorful descriptions of these places always grabbed me. Imagine, what sort of villains are so evil that Trithereon and his pack has to go hunting after them? And rather than outright execute them, they are put in their own prison plane. Trithereon's prison planes predate the Ravenloft setting as well, so ponder that. I tried to tackle this issue in a Canonfire article on his demiplanes back in 2006. I consider this article among my best work, especially considering it isn't about Ull.
So there you have it, check out Trithereon sometime when you're making a cleric for your Greyhawk campaign.