Both were once genius scientists who were putting their intelligence to use to help a morally questionable organization find new discoveries to make them more powerful (Dr. Jonathan Ohnn working for Kingpin’s gang to discover a new superpower and Dr. Robert Scranton working for the SCP Foundation to test & contain reality bending technology).
During their research, an accident started that led them to entering another reality (Ohnn going into the giant spot that spawned in his lab because a blackout threaten to destabilize it before he could take advantage of it and Scranton being transported after unexpected seismic activity damaged his lab).
They awaken floating weightlessly in a dimension with unique properties that’s essentially a world between worlds (The Spotted Dimension, a half-light half-dark world between the Dark Dimension & the Light Dimension, and SCP-3001, a parallel/pocket "non-dimension" almost completely devoid of any matter).
In this place their bodies would be completely transformed (Ohnn’s body being covered in spots and Scranton’s body disintegrating then coming back with a black acidic ooze) which gave them the unique ability of teleporting via portals that they create with their body.
However after being stuck in this reality in isolation for extreme amounts of time, they would eventually go mad and now use their abilities to sadistically kill people in creative ways with their powers.
Both would end up being imprisoned by organizations (the Spot on multiple occasions being captured by villainous groups to harness his powers for themselves and the Old Man being contained by the SCP Foundation).
Their powers have also essentially made them completely unstoppable thanks to their control over their dimension (every time the Spot dies he comes back in the Spotted Dimension and the Old Man never being completely contained by the Foundation due to his teleportation).
They do have a notable weakness that, while not ending them for good, does keep them at bay for a while (If the Spot creates too many spots it leaves himself open to attack and the Old Man’s sadism can be exploited by putting a wounded victim in his cell to trap the Old Man again).
Both are shown to have only had one person that they’ve ever had genuine love for, which is ironically what led them to killing more (Ohnn’s son, who after being shot by a gang led to the Spot going on a vengeful killing spree, and Scranton‘s wife, his main motivation for getting out of SCP-3001 then killing a bunch of people to use their vocal cords to talk to her).
