I'd say he is much much more powerful than Illidan, and would prove a worthy challenge for the Lich King.
Perfect. I see him as way stronger than Illidan (post-Ra-den empowerment), but due to his size and strenght, would be slightly close to LK. Arthas would still defeat him easily, but still.
But seriously, saying he is weaker than Illidan and comparing him to random bosses is sacrilege imho haha. Lei Shen is really strong, maybe stronger than any other mortal warrior (as in Varian, Garrosh, etc).
But when we look at the fights, well… we didn't killed the LK. He wiped us at once. Tirion didn't defeated the LK either. He, with the help of the Ashbringer, broke Frostmourne by surprise and the freed spirits attacked the LK in numbers (hundreds? thousands?). And then still, the LK wasn't defeated yet. There always must be a LK, and there is still one to keep the Scourge in check. Arthas was defeated, with quite a fair amount of luck.
Looking at the TK, not only we defeated him, without help, without legendary weapon, the TK was also using many weapons and that titan machine. And he still felt.
The LK is stronger by the look of the fights.
By all means Chuck Norris is stronger anyway.
Not just hundreds/thousands of souls. The sould of literally everyone in Lordaeron was stored in the sword, which means it probably contained several hundred thousand souls. That is a shitload of power.