The ultimate embodiment of guile and deceit, Malengin is in fact out-guiled by Artegall and Arthur on their way to see Mercilla. They hear about Malengin from Samient, a handmaiden to Mercilla, and use Samient as a decoy to entice Malengin out of his lair. Once exposed, Talus handily eliminates him.
Malengin's capture by Arthur and Artegall through suspiciously deceitful tactics points to the ambiguity of deceit and trickery, which can, in the right circumstances and under the right control, be used for good. The famous protagonist of Homer's Odyssey for example, was famous for being tricky but in a good way. So Malengin doesn't just embody and encounter with guile but a test of knowing when guile is immoral and when it is an acceptable tactical tool.