As this blog sounds like, we are looking at the powers, feats and strengths of Diablo that he has shown so far, this also includes what is demonstrated in the Lore, Cinematics and In-Game.
What I know
Diablo is perhaps the most powerful demon known to Sanctuary. and perhaps the most feared. He has the ability to influence the minds of his victims and latch onto their most crippling fears, using a person's own worst fears against them. He has the ability to turn the fears of Men into actual creatures, and even the smallest of fears provide him with power. Deckard Cain argued that Diablo would always remain the most insidious, and most powerful of all the Evils. Tyrael likewise considered Diablo to be "the worst of the Three."
Deckard Cain wrote that due to his mastery of terror, Diablo incapable of feeling fear himself. However, his confrontation with Uldyssian stands in contrast to this, realizing that Diablo himself did possess innate fear, and that he could not bear to be subjected to his own powers of terror.
In battle as the Lord of Terror, Diablo demonstrated proficiency with fire and lightning attacks. As the Prime Evil, Diablo demonstrated similar abilities, along with the ability to teleport, and to form cages of bone.
What I know
He is a master manipulator, he is capable of getting people to do what he wants to even his enemies. And no matter how many times he was defeated, he always finds a way to come back stronger than ever, even through his own daughter.
He is insanely strong, capable of breaking Imperius' spear which caused a massive shockwave. He even broke down the gates of heaven with just his roar, those are some impressively powerful lungs.
He even has crazy endurance, he can take so much damage and keep on fighting, he even was split in half and was still able to stay conscious for a while longer and even speak.