This quest is initiated by reading the book "The Legend of Motte". It is found by examining a bookshelf in Windstalker Village at -1822, -38, -542 .
You must kill 10 Highwaymen in Antonica. After doing this return to the bookshelf and inspect it.
After this you must search for Motte's Lost Tomb and inspect it (-2177, +940). You can find Mottes lost tomb in the hidden Vale. When you get to the Vale (-2050, +695) follow the left wall all the way around until you see a Den mother with her bear cubs, she doesn't aggro so you are safe to go in. Hidden vale is located at the southern most part of the map.