Lore behind it:
After killing and slaughtering countless lives throughout the omniverses, Klaus was marked and cursed by the first omnipotent being as he is now "doomed to walk the Omniverses alone for a tortured eternity", his curse made his body as immortal as his immortal soul. Even if it's supposedly 'broken' it'll eventually regenerate itself and it will once again slowly corrupt him into being jealous towards everything. It is also revealed that if the Klaus does not satisfy the Mark, by killing, it will slowly but steadily turn him into a blood thirsty monster who will be hellbent on destruction. The curse's grip affects Klaus on more than just a purely physical level, and that cutting it off or even burning it off wouldn't work and it would require a source of great power though it is only a temporary solution. The Curse's origin reaches back to the time of creation itself and that it predates the lore. The Curse imbues Klaus with several powers as well as psycho-supernatural burdens which eat away at his character.
Nothing cannot break it
No Plot Armor
No OP Omnipotent Beings
No Authors
Not anything else for that matter
Curse Effects:
Murderous Rage: The Mark affects Klaus to where he has to constantly kill something. If he doesn't, he'll eventually lose his emotions and become the very person he once was before. And if Klaus does not satisfy the Mark, by killing, it will slowly but steadily turn him into a heartless monster who will be hellbent of slaughtering for their amusement.
Self-Resurrection: In order to ensure that The Mark is always connected to him, if Klaus dies, it will resurrect him back to life making him more powerful than before because of his Saiyan biology.
Corruption: The Mark, due to its connection to the Darkness, has the power to slowly corrupt the wielder no matter how powerful a being they are. Klaus is afraid that one day, he'll snap and wipe out everything in existence and he dreads the day that it happens.
Destruction Empowerment : The most unique thing about Klaus is his ability to grow stronger and become even more tenacious with every lifeform he kills which is the result of his curse but because of his immensely strong will, he can resist this effect abit temporarily.
Master Manipulator: One of Klaus's most well-known traits during his conquest days was using his knowledge of people to make them do things in his best interests. He has a keen ability to understand the mental and emotional weaknesses, desires, and needs of his victims and determine how best to use them to his advantage.
When Corrupted) Unlimited Focus: When corrupted by the mark. Klaus lacked emotions like fear, guilt and empathy. These lacking emotions essentially made him like a machine, not experiencing the same limits a normal person has. It also increased his focus and intuition, to greater then normal levels. Making him a true monster, doing whatever it takes to get a task done at any cost, but also leaving him an emotionless and uncaring person even to those he deeply cares about. This is the mentality he once had during his early days when he was still a mortal Saiyan.
(When Corrupted) Uncontrollable Bloodlust: Even not corrupted, the mark occasionally makes Klaus hear voices in his head telling him that he is pure evil incarnate and to let go and accept who he truly is and to kill like he used to. When Corrupted, Klaus becomes a force beyond recognition because of his bloodlust to slaughter everything in his path.
(Corrupted) Hypnosis Inducement: Klaus has the ability to make people docile and susceptible to suggestion with a touch to his victim's forehead, sending a surge of energy into their minds. But due to being a Saiyan who desires a fight more than anything, he refuses to use this ability.
When Klaus is Corrupted:
The Curse's affect on Klaus will make him both highly dangerous and unstable due to the curse intoxicating him and clouding his judgment. He will begin to regress into who he was before as he was revered as the vilest and most terrifying Saiyan in existence and no other being would dare challenge Klaus for he was the strongest of them all. He often displayed a harsh, arrogant demeanor towards any lesser being. Once the curse fully corrupts him, he becomes cold, quiet, ruthless, sadistic and murderous as he kills and attacks anyone near him without mercy. While arrogant, he is not above complimenting his foes if they are truly powerful, which is a a bad habit as he likes toying around with opponents to mock them even if they proved to be very formidable.
(Corrupted) Klaus's Evil Laugh: