The Time Trapper
The Time Trapper is an enigmatic and powerful master of temporal manipulation and energy and a particular menace to the 30th century era of the Legion of Super-Heroes. First asserting himself behind the scenes, he erects an Iron Curtain of Time that prevents even the most powerful Legionnaires from traveling to their future (Adv No. 317, Feb 1964: "The Menace of Dream Girl!") - and later blocks Superboy and Brainiac 5 from returning back to the Legion's time.
Assisted by Glorith, the Time Trapper is finally defeated when his plan to devolve the Legion into protoplasmic slime goes awry, and instead, the Legionnaires become infant versions of themselves and wreck his machinery (Adv No. 338, Nov 1965: "The Menace of the Sinister Super Babies!").
The Time Trapper returns to strand Superman in an extremely distant future, one where the Man of Tomorrow must fight forward in time to finally return to the present (Act No. 385, Feb 1970: "The Immortal Superman!"; Act No. 386, Mar 1970: "The Home for Old Super-Heroes!"; Act No. 387, Apr 1970: "Even a Superman Dies!").
It is later revealed that the Time Trapper is thought to be a member of an advanced race known as the Controllers, however that is determined to be a ruse (see The Legion of Super-Heroes No. 291, Sep 1982: "A Sign of Darkness Dawning") and this imposter is rendered powerless during a sinister plan of Darkseid.