In the Classics storyline the First Ultimate Battleground is a big fight. Happens about 5000 years before Adam, after the great wars, then there's a truce and Eternia and the two towers are built.
King Powers (faceless one) breaks the truce by having a baby and war resumes. Culminates in the FUB. Depicted recently in the mini comics that came with the quarterly figures.
Major points - He-ro dies, King Greyskull dies, Adam is there in secret fighting, Zodak banishes Hsss to the void, Greyskull banishes Hordak to despondos, etc.
Second Ultimate battleground is when the towers rise from the earth in the [present time?]. Another big fight and lots of causalties.
The origin of the name comes from the mini comic that came with vintage Eternia.