That Wand of Shambling Mound is obtained by talking to/killing cultist with tamed Shambling Mound monster. If you win over some dialogue with that cultist, she can tell you how to craft it (combining branch with Tenebrium) and how it works (it gives you Rot when used without Tenebrium skill, but you can cure Rot easily with some Bloodstone).
That Guardian is visually very similar to Shambling Mounds, that's how i figured out to use it on him.
