Alex Luthor

History

Just prior to Earth-Three being destroyed by a wave of anti-matter unleashed by the Anti-monitor, Earth-Three's Alexander Luthor placed his infant son, Alex, into a dimensional top and sent him hurtling through the universes to Earth-One.

The top spun through the anti-matter wave, imbuing the child with both positive and anti-matter energy, making young Alex a living matter transformer.  Arriving on Earth-One, Alex was brought aboard a satellite belonging to the Monitor, where Alex quickly changed from infant to child to teenager.

There, Alex discovered his powers. Able to summon his anti-matter abilities, the youth could bridge universes from positive matter to anti-matter, and he could use himself to let other positive matter beings enter the anti-matter universe without harm.

At the end of the Crisis on Infinite Earths, Alexander Luthor, Jr. opened a one-way portal to an unknown "Paradise Dimension", which allowed the Superman of Earth-Two, his wife Lois Lane and Superboy of Earth-Prime to be saved, along with himself, surviving the annihilation of the Anti-Monitor. The recent Infinite Crisis has seen a number of those characters return to the mainstream of the DCU as the Multiverse appears to have returned.

The Joker kills him with slight help from Lex Luthor. Alex Luthor has been identified as one of the deceased entombed below the Hall of Justice.
 

Blackest Night

During the Blackest Night, Alexander Luthor was raised by a black power ring and became a Black Lantern. He tried to kill Flash, Atom and Mera in the Headquaters of the Justice League.  He has since used several black power rings to warp into the Multiverse to attack Superboy-Prime on Earth Prime.