Oy. Those spirally boils all over that thing's body are kind of gross. The realistic eyeballs and exaggeration of magcargo's melty mouth give a great frightening effect. I guess giant snails are kind of frightening in their own right, so enormous snails made of lava are naturally even moreso.
Though I would have liked to see this one with a flaming radula.
I like the kind of stylistic way you drew its plume of flame on the top. The spirally lines in between give a pretty neat effect. I also think the bloody lines in the background go well with magcargo's red colouration. Good work on this menacing mollusc.