Here are the fact:
C'Thun is the weakest Old God in Azeroth. With the Prophecy of C'Thun tell us that there was a fight between a titan and C'Thun. C'Thun was able to kill the titan he was fighting against. It did weaken him to a point that the other titan thought he was dead.
If you look at the info, it took a group of titan to bring down one Old God.
Let look at the power level of each Old God.
5. C'Thun ("Killed")
4. Unknown (Killed)
3. Yogg-Saron (Imprison)
2. Unknown (Imprison)
1. Nightmare God/Maelstrom (Imprison)
If you read the novel, War of the Ancients, there were only three that were imprison. Those three are the most powerful Old Gods of Azeroth. At the same time in the novel, Imprison Old Gods did said that Sargeras is nothing compare to them. Sargeras is walking into a trap, it is likely that Nightmare God/Maelstrom Old God is the one that would kill the dark titan once the portal been entered by Sargeras. Elemental of surprise would been the major player to the death of Sargeras.
Sargeras did not know or have the time to fight back against the Old God.
Since we have the first well destroy, I would said that Nightmare God/Maelstrom Old God been active along with C'Thun. Yogg-Saron had to wait until the Night Elves planted a second world tree to break free. The Unknown Imprison Old God might been active as well, where you can read in the novel, Day of the Dragon.
Due to Harbinger Skyriss, we can said that there is more than 5 Old Gods. The Armies of the Old Gods is bigger than Burning Legion. Nathrezim could have gain their power to corrupt from the Old Gods that are outside of the Azeroth. Making it the Old Gods having a connection to the corruption of Sargeras. We can also said that the Elemental of Azeroth were special to the Old Gods and could deal with the right hands of Sargeras, Kil'jaeden the Deceiver, and Archimonde the Defiler.