A weapons designer who built machines to hunt mutants, and whose death would have made them unstoppable.
Bolivar Trask is the man who invents the Sentinels, the robots that hunt mutants in the Fox X-Men films. Peter Dinklage plays him in X-Men: Days of Future Past, as a scientist who is entirely sure his work is good for the world, which Dinklage described as the core of him: drive, blinded by arrogance.
The film turns on him. In its dark future of 2023, the Sentinels have nearly wiped out mutants and the people who protect them, and Xavier traces it back to a single day in 1973, when Mystique assassinated Trask. His death got the program approved, and her capture gave it her DNA, which is what let the Sentinels adapt to any mutant power. So Wolverine is sent back to 1973 to stop the killing, not to stop Trask.
In 1973, Mystique has found out that Trask has been experimenting on mutants, and goes after him at the Paris Peace Accords. The attempt fails, mutants are publicly exposed, and Trask uses exactly that to get President Nixon to authorise the Sentinels.
Minor spoiler
At the unveiling in Washington, Mystique gets her chance again, and Xavier talks her out of it. The country sees a mutant save the President, the Sentinel program is shut down, and the dark future never happens. Trask is later arrested for selling military secrets to foreign governments.
Show detailsAn earlier Trask
X-Men: The Last Stand has a character called only Trask, the head of the Department of Homeland Security, played by Bill Duke. Wikipedia’s article on that film calls him probably related to the comic books’ Bolivar Trask, but notes that the film never gives his first name. Wikipedia’s list of X-Men film characters, read the same day, marks Trask as only mentioned in that film. This entry follows the narrower reading and keeps Days of Future Past as his only appearance, but the two pages do not agree.