Not just that, he also had a few death entities totally subservient to him, all of them powerful in their own right.
Mueh’zala considered Zovaal’s power to be worth more than anything he could ever get from worship and sacrifices, Helya was subservient to him also, though i don’t know why Helya was convinced by Mueh’zala to serve Zovaal. Likely promises of power against Odyn.
Mueh’zala lacks any notable feats of strength as far as I am aware, but he was capable of elevating his followers into Loa themselves, and he also held the power to one-shot the party in De Other Side if they didn’t get to the death gates fast enough, a spell called “Shatter Reality.” Other than that that, he’s still the OG Troll
Loa of Death and should logically have the experience and power to boot.
Helya was a Titanforged sorceress that was forced by Odyn to ferry Vrykul souls from the Shadowlands to produce Val’kyr, in addition to turning any unwilling Vrykul souls into Val’kyr. Her power wielded by her followers was enough to where she could turn Odyn’s Stormforged into Kvaldir. She was strong enough to assist Highkeeper Ra in crafting the Elemental Planes, and she was strong enough to personally forge her own realm: Helheim. She was still completely loyal to Zovaal.
Sire Denathrius, an Eternal One just like Zovaal created Revendreth, the Venthyr, and the Nathrezim. Furthermore, with the power from the Medallion of Dominion, a fraction of Denathrius’s power, Kel’thuzad could raise up an unwilling army of Revendreth’s denizens, Renathal even theorizing that he could alter the very fabric of the Shadowlands with enough anima. Denathrius was still subservient to him, and was weaker. I’ll get to why.
The Eternal Ones had to team up in order to defeat him when Zovaal first revealed his betrayal. The Winter Queen, who had made the realm of Ardenweald by herself, and it was through the power of Ardenweald’s Heart of the Forest that the Maw was inescapable. The Primus had made Maldraxxus, the birthplace of necromancy and the Necrolord Covenant. He also made a rune language to empower the Maldraxxi even further, this same exact language being the one that the Death Knights utilize in runeforging. He was said to be a master tactician, strategist and overall was a S++++ tier commander. The Archon, Kyrestia (presumably) made Bastion and was the first of the winged Kyrian, she was also an Eternal One. Denathrius joined his fellow Eternal Ones, and it was only because they worked together that they defeated Zovaal.
When the Primus went to engage Zovaal by himself, he lost. Considering that he lost, it’d be more than likely if not guaranteed that the other Eternal Ones would lose in a one-on-one fight with the Jailer, simply because he holds more raw power than any of them individually. (Or well, two v one technically, as Remornia is Denathrius’s sentient weapon. )
I’d also generally apply this to the Mawsworn army too: The Shadowlands Covenants had to unify and invade the Maw as one, working together to breach Zovaal’s strongholds in the Maw alongside the Knights of the Ebon Blade and the Maw Walkers and disrupt his operations in Korthia. Individually, the Mawsworn’s army would be stronger than the other Covenants.
TL;DR: Zovaal’s Titan++ status was legit, and it was through sheer plot that we won.