I show it again for you just tell me The True Lichking after fusing with Nerzuhl would still loose to Deathwing..
„ Arthas placed the unimaginably powerful helm on his head and became the new Lich King. Ner'zhul and Arthas' spirits fused into a single mighty being, just as Ner'zhul had always planned. Illidan and his troops were forced to flee back to Outland in disgrace, while Arthas became one of the most powerful entities the world had ever known.“
Ner'zhul's spirit was placed within a specially crafted block of diamond-hard ice gathered from the far reaches of the Twisting Nether. Encased within the frozen cask, Ner'zhul felt his consciousness expand ten thousand-fold. Warped by the demon's chaotic powers, Ner'zhul became a spectral being of unfathomable power
.... To this end, Kil'jaeden called upon his elite demon guard, the vampiric dreadlords, to police Ner'zhul and ensure that he accomplished his dread task.
Tichondrius, the most powerful and cunningof the dreadlords, warmed to the challenge; he was fascinated by the plague's severity and the Lich King's unbridled potential for genocide.
And still the Lich King was smarter.
His psychic powers proved to be almost limitless, and he used them to create a small army that he housed within Icecrown's twisting labyrinths
There were a handful of powerful individuals scattered throughout the world who heard the Lich King's mental summons from Northrend. Most notable of them was the archmage of Dalaran, Kel'Thuzad
He is the one who is manipulating, he is not a victim like Deathwing who got corrupted, he corrupts
This is all statements from the ingame books of WoW Classic.
It was the way, blizzard gave us more background information.
I dont call The Lichking as the strongest being, i beleive the intending idea and description is showing us @ wich power level he really was.
Maybe he would loose maybe not i domt beleive deathwing would has an easy fight..
Just watch night king vs drogon