I believe that Odium has already agreed to a contest of champions but that contests itself is unresolved. I was listening to e Odium Shardcast, and they were asking basically "what would prompt Odium to agree to it" first I think that they covered some key points from which the most relevant to me I believe it is the fact that going against two shards that are cooperating was extremely risky for him, and I also believe that Tanavast may have given himself impossible odds. I think that the original contest of champions had something to do with the Heralds. I am pretty new to Stormlight so the timelight is a bit fuzzy, but here is what I imagine happened.......Honor saw that keeping Odium trapped in Roshar was something of a sacrifice to protect the rest of the Cosmere which is an Honorable thing to do, so he made a sweet deal for Odium to take that was basically the Desolations in which Honor and Odium would choose Champions, I am certain that Honor's were the Heralds, and maybe Odium's were the Unmade, but I am not sure on that second one. Anyways it was a fight to the Death, and would not be resolved until either group was destroyed. Since the Unmade are very hard to kill Odium jumped in on the deal, but then Honor went and did the Oathpact, essentially creating a loop of desolations that would essentially trap him.
Then I think that in an attempt of tipping the scales Odium started creating the Fused, then Honor started the Heralds as a response. I think that after a while, Odium got frustrated and either went straight for tanavast, or just went straight for roshar which ended up on Tanavast's death and essentially leaving Odium trapped.
On shardcast they also covered the fact that Odium recognizes that leaving Honor's splinters to freely roam around on Roshar wold be a problem for him on the future makes sense since he was unable to do anything else after killing Tanavast since he was and is still bound by the initial contest. I believe that this makes sense given what he is having Moash do by the end of OB.
I realize that there may be a lot of flaws with what I just wrote, but that is kind of what I have imagined happened; I just find it easier for me to continue with the story if I have some sort of plausible explanation of why we got where were are on the books.