Knull, aka the King in Black, has quickly established himself as one of the most powerful beings in the Marvel Universe, capable of enslaving Celestials and decimating even the most powerful heroes. His suite of powers is vast, but what exactly are they?
Created by writer Donny Cates and artist Ryan Stegman for their run on Venom, Knull is the god of the symbiotes, and he can create and control them - a significant power when the symbiotes themselves can control other life. This is part of a larger power that allows him to manipulate the Living Abyss, the embodiment of the darkness that existed before the universe. Being a god, Knull has super-human strength and durability, and he is incredibly hard to injure, although it can be done. There has been some speculation over whether Knull can truly be killed, as he has been hit with the Silver Surfer’s Power Cosmic and lived. The symbiotes he controls have shape-shifting powers, and Knull has this as well. Knull also brandishes the Necrosword, forged from the primordial darkness, and this sword has beheaded Celestials. In short, Knull is powerful, though he's also far from omnipotent.
Knull's powers have been on full display since his debut; one of his earliest appearances, Silver Surfer: Black (also written by Knull co-creator Donny Cates, with art by Tradd Moore) shows this. The Surfer encounters Knull at the dawn of time, and Knull instantly tries to enslave him, forcibly bonding him with a symbiote. To be able to do this to the Surfer is no small feat. Knull also demonstrates energy projection, combating the Surfer with a purple, flame-like aura, though this doesn't appear to be his go-to weapon. Once freed from Knull by a younger Ego the Living Planet, the Surfer hit him with the Power Cosmic, arguably one of the most powerful forces in the Marvel Universe, and while it did hurt Knull, it did not kill him, demonstrating his durability. Knull appears to have fully healed since, and has survived in space unaided, suggesting he has no need for traditional sustenance, and may not tire naturally.
After almost two years of build-up, Knull finally comes to Earth in King in Black #1. Many of the powers he displayed in Silver Surfer Black were seen here, such as symbiote control, shape-shifting and superhuman durability. Yet when the Sentry attacks, we get to see another power: Void manipulation. Deep inside the Sentry lives a being of pure darkness, and Knull was able to call it out, ripping the Sentry, one of Marvel’s most powerful characters, in two. Finally, Jason Aaron and Esad Ribic's exploration of Knull's Necrosword in Thor and King Thor has shown it alone can lend its wielder godlike strength and durability, though it also has a corrupting effect, similar to the bond between Sauron and the One Ring in Lord of the Rings.
In just the handful of appearances Knull has made, he has shown himself to be a truly powerful antagonist, able to kill Celestials with one blow and rip powerful post-humans to shreds. His power set makes true defeat highly unlikely, so watching the heroes figure out how to banish him will make for excellent reading.