Mister Eight is an alias occasionally used by Terry Sloan, of Earth-2. The character is based on Terry Sloane, the original Mister Terrific. This Golden Age hero first appeared in Sensation Comics #1 (Jan. 1942), and later joined the Justice Society.
In The Spectre v.3 #54 (June 1997), a new Mister Terrific was introduced: Michael Holt. Holt is also a prominent Earth 2 character.
Terry Sloan is a scientific genius with questionable motives and a suspicious origin. (Sloan designed special glasses that could identify people from Earth-2. When Michael Holt used the glasses to view Sloan, he was identified as not an Earth-2 native. NOTE: This story was set in a possible future.)
Sloan worked frequently for the military. He was the first to locate the Kryptonian vessel containing Val-Zod. When the boy arrived on Earth, Terry tracked him down and hid Val from the world. Val was kept in seclusion, which encouraged his agoraphobia, and essentially kept a Kryptonian "weapon" in the World Army's back pocket. (Earth 2 #22)
At some point, Sloan was inspired by Earth's "wonders" to don a costume and call himself Mister Eight (the "eighth wonder" of the world). Mr. Eight joined Superman, Batman and Wonder Woman as their main strategist in the war against Apokolips. Terry fought with these heroes for three years, but always sensed that his own agenda diverged from theirs. These beliefs grew from his discovery of the Ninth Dimension, where he was privy to visions of possible — sometimes horrifying — futures and of alternate realities. Sloan made the decision to take drastic action, if needed, to avoid them. In a critical mission to destroy a "rapture generator" (for Darkseid's Anti-Life mind control), Mister Eight made his boldest move. Instead of trying to free the areas under Darkseid's control, he decided to destroy them. He formulated an opal kryptonite which drove Superman mad, and into combat with Wonder Woman. Batman nearly stopped him with a bomb (which left Sloan scarred) but he succeeded in destroying the affected areas of the planet. This left the Earth with several five-thousand-mile burning craters — a look not dissimilar from Apokolips itself. Sloan retreated to the Ninth Dimension and never encountered the Ternion of heroes again before their untimely ends. (Earth 2 #0)
In time some came to regard Terry Sloan as a hero, and others as a genocidal traitor. Regardless, his intellect was in continuous demand. Alan Scott, president of the Global Broadcasting Company hired Sloan to develop a broadcast technology that could reach across multiversal boundaries. Sloan accepted, but he had his own reasons for wanting to develop this technology. He had already glimpsed other worlds and discovered Michael Holt (Mister Terrific). Sloan commented that he'd like to "add [Holt] to his collection," and suggested that he'd already met other counterparts on parallel worlds. Scott's partner, Sam Zhou, warned him not to trust Sloan, and Zhou leaked information about the project to Accountable (aka Jimmy Olsen). Before he could release the information, Olsen was captured by Batman and Robin. Sloan learned about the betrayal and hired a hit on Zhou. (Earth 2: Worlds' End #1)
Alan Scott and Sam Zhou were on-board when Sloan's operatives bombed a speeding train in China, killing Zhou and everyone else on board — except for Alan. Scott was protected by "the Green," an elemental Earth force that transformed him into its champion, the Green Lantern. Back in America, Terry Sloan welcomed his Earth-0 counterpart,,Mister Terrific, to Earth-2. Holt had no time to react before Sloan incapacitated his T-spheres with a high tech "pixie dust." (#2)
The threats from Apokolips were never fully defeated and the governing Council of the World Army chose to enlist Terry Sloan's aid in beefing up their super-human defenses. The Army's Commander Amar Khan strenuously objected to using Sloan, but he was overruled. (#5)
Sloan wasted no time. With the help of Dr. Robert Crane and Gen. Sam Lane, he finished the android Red Tornado and uploaded the memories of Lois Lane into it. (#5, 7, 15, 17) For unexplained reasons, he sent Captain Atom to retrieve the young Henri Roy, Jr. from Saigon. (Earth 2 Annual #1)
Meanwhile Khan assigned his black ops Sandmen to locate Sloan's hidden bases. The Sandmen discovered Michael Holt in one of them; he was brainwashed and defended Sloan's lair. (#7)
When Steppenwolf launched a full-scale attack, Sloan and Holt were scooped up by the insidious mentalist called Bedlam. Bedlam conscripted the men's talents for building a gateway large enough to move Earth into the same universe as Apokolips. (#21) The scientists actually succeeded in building Bedlam's portal. But as they opened it, Earth's heroes defeated Bedlam's allies and Bedlam momentarily lost his mental dominance. Bedlam managed to regain control over them, but it was short-lived. (#26-27)
With the help of Mister Miracle, Sloan bested the villain by entering his mind, fighting his own demons, and shutting Bedlam down. (Earth 2: World's End #2)
Terry Sloan has no metahuman powers. He is a technological genius across many fields, including robotics and quantum physics. He uses his exclusive access to the Ninth Dimension as a retreat, and for monitoring events on other worlds, and even the future.