Up here is my conversion of a chaos warmammoth. It looks brutal and towers over the battlefield, very imposing. It's made of a Thundertusk, Warshrine, Blood Throne and some various knight, blood reaver and chaos lord bits. Model is covered in savage orruk arrows, with extra armor from chaos shields. The throwing spear marauders are made from Blood Reavers and Marauder Horsemen spears, while the beast master is made from a Chaos Knight and some Skullcrusher ride bits, with a whip from a Bloodstoker. The Chieftain is a cut and trimmed Manticore Lord made to sit on the throne. The Howdah is the back part of a Warshrine, with the skull pit slightly covered with a blood throne.
Anyway, worked out a lot better than I thought it would, but I imagine the real War Mammoth from Forgeworld would have been at least 3 times as big.
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