A warp spasm is somewhat like when a dwarf currently goes berserk but certainly more dangerous and visually shocking. It comes from Irish mythology and happens during battle in times of stress, rage and sometimes need. It causes those under its influence to physically warp, hold off whole sieges and kill hundreds with ease and still not think it too many.
Here is a description of what its like.
The first warp-spasm seized Cúchulainn, and made him into a monstrous thing, hideous and shapeless, unheard of. His shanks and his joints, every knuckle and angle and organ from head to foot, shook like a tree in the flood or a reed in the stream. His body made a furious twist inside his skin, so that his feet and shins switched to the rear and his heels and calves switched to the front... On his head the temple-sinews stretched to the nape of his neck, each mighty, immense, measureless knob as big as the head of a month-old child... he sucked one eye so deep into his head that a wild crane couldn't probe it onto his cheek out of the depths of his skull; the other eye fell out along his cheek. His mouth weirdly distorted: his cheek peeled back from his jaws until the gullet appeared, his lungs and his liver flapped in his mouth and throat, his lower jaw struck the upper a lion-killing blow, and fiery flakes large as a ram's fleece reached his mouth from his throat... The hair of his head twisted like the tange of a red thornbush stuck in a gap; if a royal apple tree with all its kingly fruit were shaken above him, scarce an apple would reach the ground but each would be spiked on a bristle of his hair as it stood up on his scalp with rage.
—Thomas Kinsella (translator), The Táin, Oxford University Press, 1969, pp. 150-153
The above just before he kills hundreds in this form.
Although I realise that battles are currently fairly easy and having this happen, even only sometimes, would just make them easier I hear that battles will be held on a grander scale at some point and it just might fit then. On a similar note I'd love to read about such events in legends mode, could even get creative with the descriptions of the warp itself. In adventure mode you might even lose control of your adventurer as he rampages.
I should probably have developed this idea further before suggesting it but I cannot think of anything more to add.
edit to add, having been warped and killed many they turn back to normal, this new form isn't permanent.