all right, from my knowledge Illidan goes for melee in-character, which he can contend with and surpass characters who should have more experience in melee then he is, also Illidan like all Demon Hunters can walk into a horde of hundreds of demons and come out unscathed so Sigvald and Illidan should be about even in terms of skill.
in terms of AP Illidan scales to stomping characters who upscale from Baseline, being comparable to a Pre-lich king Arthas but Sigvald should still have AP in the bag.
also even though Illidan seems ridiculously experienced he spent the majority of that 10000 years rotting in a jail cell, he only really has at most 100 to 200 years of combat experience, granted most of that was fighting characters comparable to him in skill and hordes of demons so theres that.
as for the fight itself Sigvald's morphing weapon is going to catch Illidan off guard the very first time it happens, basically if that doesnt kill Sigvald now has to deal with Fel Magic range spam which includes AOE, with Metamorphosis Illidan certainly has the AP advantage, of course Illidan's two auras are certain to throw Sigvald into his brute force rage, which will utterly backfire due to Illidan's experience, considering multiple demons enjoy trying to brute force their way through everything and Illidan basically invented the art of fighting demons, of course very little of Fel Magic hax actually does anything of note even to those without resistance to it so Sigvald doesnt have to worry about getting haxed to death, he has to worry about getting outranged and his own tendacy to brute force when he takes any damage to his face and body, also the fact that Illidan is really good at dodging attacks from characters comperable to him in speed.
basically Illidan holds most of the cards for his in-character fighting style being one that doesn't screw around, Sigvald's only real chance at killing Illidan before he is overwhelmed is when he first uses his shapeshifting weapon to his advantage, if that doesnt kill Illidan, Sigvald is going to have a tough time so much as touching Illidan when the ladder is playing keepaway due to Illidan's ranged options and ability to fly.
i think this goes without saying but Illidan's immortality and Regeneration are not combat acclible, they take years to actually go through.
