I am the real god of all!
He's a big edgelord and he's cool, I guess. Apparently he's going to get more feats in the future, he's getting hyped again for something. He's pretty strong, and seems to base himself on the manipulation of darkness, and the species that he created It out of.
In terms of physical stats, he doesn't have a clear level yet, but appears immensely powerful.
Tanks and survives the Big Bang, taking millions of years of energy in the process. (Venom #4)
Cates himself (the writer of VENOM and creator of this character) states Knull is unkillable, and King Thanos didn't destroy him in his timeline, so that's a plus, I guess.
Eddie implies that Knull is more powerful than all of Earth's main teams combined. We'll see if this is still true when the rest of the run finishes. (Free Comic Book Day 2019- Venom-Spider-Man)
A tiny bit of Knull's essence is able to control Carnage. (Web of Venom- Carnage Born)
After hiding inside another Symbote, he rips Eddie's off with ease. (Venom #6)
Casually tanks Venom's blows by using a non physical vessel, and easily removes the symbote using his powers. (Venom #3)
Creates a giant arm out of a Symbote and crushes a helicopter with it. (Venom #3)
Summons and controls a giant building tall Symbote that can no sell tank fire. (Venom #3)
Traps Eddie inside another symbote controlled by him. (Venom #3)
He loses his full control over all Symbotes who were all over the galaxy after Thor severed the connection between him and them, making them seek out random hosts in the process. His current physical form is trapped under billions, if not trillions, of them consistently making sure he doesn't escape as he is still needed to prevent them going mindless. (Venom #4)
Was stated to be the god of the Exolon (another species of parasites) showing him to be the creator of them as well, likely through the same process as the Symbotes were created. (Guardians of the Galaxy #3)
Creates a sword out of the Void, using it to kill a Celestial in one hit. This blade would later be known as All-Black, or the Necroblade, the first Symbote ever created. (Venom #4)
With the creation of All-Black, he killed a billion stars worth of gods. (Venom #4)
His former weapon forms with a young Gorr and gives him the capability to become the God Butcher in the process. Basically, Knull is the rightful owner of the Necroblade, and seems to be much better in using it. (Thor- God of Thunder #6)
By manipulating the Abyss and bonding it with any other creatures, he found that he could control them, drain their energy for himself, and learn all of their knowledge in the process. This creation becomes the consensual symbote, and becomes a ungodly massive army that goes through half of the galaxy and destroy entire civilisations. (Venom #4)
He's got a lot more feats to still occur, to be honest. Based on what already has been seen, he seems to be pretty powerful, but until we get to see more of what his actual physical body can do, it seems to me that he's just ungodly durable.
Oh, and his summons are pretty powerful as well, I guess