Who's This? The wizard on page 18 of Who's Who vol.XXV.
How you could have heard of him: The Warlock has resurfaced since in modern JSA comics, first in JSA #54 (2004), allied to another also-ran Kulak; and 2008's Brave and the Bold #9-10. He was glimpsed in the New 52 as one of the interdimensional beings imprisoned by A.R.G.U.S. in Vibe's series. Otherwise, he's mostly been a cameo in accounts of Zatanna's history.
Example story: Green Lantern vol.2 #42 (1966) "The Other Side of the World" by Gardner Fox, Gil Kane and Sid Greene
It all begins with a weird moment: Hal recites his oath and it comes out of his mouth in Zatanna's reverse speech. Still, the ring charges up fine, and off he goes to fight a big pirate with magic powers called Patch-Eye Pete. During the battle, Hal's bacon is saved a number of times by the ring exhibiting unusual abilities, including some power over yellow objects. Well, it's all thanks to Zatanna who cast a spell to lend GL some of her magic.
The Warlock of Ys could have had more appearances. I mean, it's not a bad look, and a wizard trapped in another dimension, but sending armies from across history across the threshold is a concept that could have worked well with a variety of heroes, or even the Justice League itself. Maybe giving him a page in Who's Who was a way of repairing the oversight, but no one really took the bait...
Who's Next? Before leaving volume 25, we simply must take one last dip in the Golden Age.