The Mistress Death
The death that is shown throughout marvel comics is the anthropomorphic version of the actual concept of death. However, she is only a part of Oblivion and the mere absence of her presence defies Oblivion.
While not as powerful as Oblivion, Death is actually a key element for TOAA and the marvel creations. The Marvel corporation allows stories and get entertainment, artistic value and profit because Death was pitched to the board. However, if the executives feel like it, they can cancel the ideas any time.
Thus, even Death can die, which is later represented by the Death of Death, an aspect of The Living Tribunal.
Death's domain is outside of space and time.
Death exists outside of paradoxes, and in fact, even exists in the Mystery. As such, Death can kill beings even if they are outside of death (yeaah, i know. comics get ridiculous sometimes):
Is implied to be able to give Mephisto true death.
Kills all the cancerverse characters and then the entire cancerverse, all of which were outside of death.
A weakened and small portion of death straight up kills Walker in Purgatory (a pandimensional place where no one is supposed to die and death had no dominion there). Then leaves the pandimensional place before anyone notices.
Her awareness is so fine tuned she even knew that which was scripted to happen by TOAA. Something that no one else except TLT knew.
She can even take true platonic archetypal beings.
Thanos, empowered by death, beats the shit out of Eternity and later matches enigma force Deadpool until death takes away her amp.
Her amp also allowed Deadpool to tank Eternity's attacks.
Death travels from space to her dimension within an instant from Thanos' field of vision.
Constructs an infinite building without end where each room of each floor is a metaphysical construct of different afterlives. This building exists in the true White Hot Room as well.
Death is beyond the controls of heaven and hell.
Full power of classic Doctor Strange does nothing to Death. Incapacitates Strange by merely pointing her finger at him. According to Strange even Dormammu, mephisto and the ancient one weren't even this powerful.
Also, Death can become someone's thoughts itself.
Turns Silver Surfer, Nova and even Galactus into plasmoid blobs.
Straight up yanks the Phoenix force out of Rachel's body and forces it to a timeless limbo.
