Tom and his cousin Sean (Banshee) were rivals over a woman named Maeve Rourke. Maeve married Sean. However, her death seemed to revive their dislike for each other, not to mention Tom secretly raising Sean's daughter Theresa.
The X-Men initially clashed with Tom and Juggernaut at Cassidy Keep. He and Juggernaut were involved in many schemes. On one occasion Juggernaut sought to share his power with Tom, in essence creating two Juggernaut-like beings.
Tom somehow contracted a type of plant mutation. He sought to use Deadpool's healing factor as a cure. He later attacked Sean and Generation X in a partially transformed state. He was defeated by Banshee, and the source of the plant was removed. However, when the X-Men later encountered Tom as part of the Brotherhood, he had become fully plantlike. Like the rest of the Brotherhood, Black Tom was sucked into Shen Xorn's head and transported to the Mojoverse, where they sold Juggernaut and Nocturne to Mojo and, in return, they were sent back to Earth.
He blasted Dazzler of Excalibur, using technology to replicate his powers. Tom has since shown remorse for the death of Squid-Boy and claimed to have had his mind affected by the plant mutation. He also urged Juggernaut to recover his lost strength and return to his full strength, feeling that Juggernaut couldn't become reformed and pointing out that Juggernaut had yet to tell his new allies of his full past.
He was seemingly repowered, using his powers during an encounter with Deadpool.
Black Tom is a mutant, who was seemingly depowered on M-Day, replicated his powers via technology and then was somehow repowered. His powers included:
Bio-Organic Thermokinetic Blasts: Cassidy has ability to generate blasts of concussive force or heat by using wood as the focus for his power; typically a wooden shillelagh. Following genetic engineering, Cassidy can channel the energy he generates directly through the parts of his body that were grafted to a wood-like substance.
Healing Factor: Cassidy could regenerate damaged or destroyed body parts with great speed.
Chlorokinesis: He could also mentally control all plant life within his vicinity.
Plant Consciousness: He could distribute his consciousness amongst plants he controlled.
Life-Force Absorption: Cassidy could drain the life force of organic beings.
Possible Genetic Immunity: Black Tom is highly resistant, if not immune, to Banshee and Siryn's sonic screams, and they are similarly protected from his bio-blasts; which is similar to Cyclops and Havok's sibling power defunction to each other.