The steel breastplate is located in the tunnels of Haven underneath Eternia in the possession of the Valkyrie.
The quest:
From Cliff Face: n, sw, w, swim sea, 2e, ne, 2w, n, ne, 4e, n, 6e, s, 2e, d, 2s, e, sw, s, 2nw, e, crevice, crevice, d, e, d, ne, n, d, 2s, 2w, 4s, w, stairs, get sceptre, kill valkyrie, get breastplate from corpse.
Tips:
At the time that you would normally go get the breastplate, the valkyrie shouldn't pose any real threat. However, if you're a ranger, I'd recommend waiting a few more levels and going after Thaolan in the forest of Xerxes and taking his Scales of the Alda.
