Cross Island was a cross shaped peninsula on the southern shore of Lordamere Lake. It was the site where the Violet Citadel of Dalaran, stronghold of human mages, was located — which was outside the city of Dalaran. It was rebuilt on Cross Island after it had been sacked during the Second War. The orcs tried to attack it again in order to get the Eye of Dalaran.
By the time of Warcraft III, a portion of cross island may be visible in two of the level maps. However, no island appears to be visible in the cutscenes or loading maps. It may imply that the nearby lake has receded, or land was built up around Dalaran, so that the land became part of the main shoreline. In fact, there is some implication to this according to the maps themselves in that the Beyond the Dark Portal map shows very little water. Fenris Isle may be what is left from Cross Island. Some parts of the island have seemingly sunk. Alternatively it could just be a retcon.