hi there bloodsuckers iv just bought myself a mortarch but it was the age of sigmar version and i want to put it on the fantasy base, is it the large monster base (like the arachnarok and terrorgheist)?
Yeah, it's the 60mm x 100mm, same as the Hell Pit Abomination and the Screaming Bell. When making one, the Easiest thing to do might be to take a bunch of 20mm squares, glue them to a piece of styrene card in a 3-wide by 5-deep layout. Trim off the excess card fill the gaps between the bases with putty. This will give you the same bevel as the other GW bases. Then, use a sanding stick to sand the edges so the base plastic and putty is smooth and uniform.