Lord Elgnub(gnoll slayer) spawn? how long?

[iasus]

Like the title says. I've been sittin here in blackburrow for 8 hours (real time). Haven't seen him spawn... been killin every snake(even by ladder) and clearing every commander/elite, split paw commander room over and over. How rare is this guy? and is there a way to trigger him I don't know about?
Help appreciated thanks!