Mind, body, and soul.....
Mastermind was for the most part a jobber. Started off as one of Magneto's henchmen and spent decades as a boring side character with one dimensional plots and a generic personality.
Then Claremont happened. For some reason, everything he touched as a writer became golden and as a result, Mastermind became a complex and interesting character, rich with personality within the Phoenix and Dark Phoenix sagas (which are both amazingly written and stand out as masterpieces of the comics genre) and became one of the best known villains of the X-Men.
He is also one of the few mutants to not be brought back at any point since his death in the 90's, being succeeded by his daughter. He's stayed dead, and as a result, hasn't gone down the lame route like Colossus or whatever and kept getting brought back to life.
dodges a laser blast while casting his illusion. (Uncanny X-Men #6)
Dodges a optic blast and then creates a illusion that blinds people. (Uncanny X-Men #5)
Handbook on his capabilities. Bit wordy, but gets the point done. (Official Handbook of the Marvel Universe)
Creates a entire army out of illusions that take over a city long enough for Magneto to create a real army. (Uncanny X-Men #4)
Beast knows that Mastermind's illusion isn't real but recoils and reacts regardless. (Amazing Adventures #13)
Convinced Beast that he killed Iron Man and gave him amnesia. (Amazing Adventures #13)
Creates a massive illusion to a crowd that Cyclops is attacking them. (Uncanny X-Men #134)
Creates multiple versions of himself. (Uncanny X-Men #5)
Edits the memories of Polaris as a child to misremember her parents death. He states that he can't do this to older people. (X-Factor 2012)
Fools the X-Men, including Jean Gray, into thinking they were trapped. (Uncanny X-Men #4)
Incaps and traps Iceman and Cyclops, fooling them into thinking their powers are useless. (Uncanny X-Men #39)
Influences the dreams of Rachel Gray and edits her memories into thinking that Franklin survived. (Excalibur #175)
Uses multiple illusions and manages to hide from Phoenix Jean Gray's casual telepathy. (Uncanny X-Men #125)
Turns a stone wall to glass, fooling Beast into thinking that he can't grab on to it. (Uncanny X-Men #4)
Is able to hide his thoughts from Phoenix Rachel Gray and convince her to give him some of her power via a telepathic link. Mastermind managed to do this while hiding his real intentions. (Excalibur #175)
Beast is unable to move, even when he knows that it is a Illusion. (Uncanny X-Men #5)
Manages to incap Captain Britain by leaving him in some state of inactivity within his mind. (Excalibur #175)
Fools the entire X-Men into fighting Cyclops by tricking them into thinking he was Dark Phoenix. This was also realistic enough to fool Wolverine's amazing senses, which can usually detect illusions or falsehoods. (Uncanny X-Men #175)
Fools the X-Men into thinking they are fighting Dark Phoenix, including Logan. Should be noted that there is a physical person there, but they have been fooled into thinking that they are Phoenix and doing all of this. This is one of Mastermind's best and most complex illusions. (Uncanny X-Men #175)
Messes with Sentry's mind (regardless of his amazing telepathy and defences) and literally invents The Void as a concept. (New Avengers #9)
A dying Mastermind from the Legacy Virus is able to overcome the likes of Jean Gray and Iceman and places them into his illusion world, as well as bypassing her telepathy. (Uncanny X-Men Annual #17)
Fools the X-Men into thinking that New York has been destroyed. Should be noted that his illusions kill Cyclops for a while, showing that they are so realistic that the human body can die as a result because it thinks it is getting attacked. (Uncanny X-Men #175)
Not a lot to talk about, but Mastermind is pretty powerful close to the end of his life. His telepathic defences are amazing, able to bypass the likes of Jean and Rachel Gray.
His illusion creating is near peerless in terms of street level characters.