Hermann, Pernille, Stephen A. Mitchell, and Jens Peter Schjødt, eds., with Amber J. Rose. 2017. Old Norse Mythology—Comparative Perspectives. Milman Parry Collection of Oral Literature 3. Cambridge, MA: Milman Parry Collection of Oral Literature. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_HermannP_etal_eds.Old_Norse_Mythology.2017.
Óðinn, Charms, and Necromancy: Hávamál 157 in its Nordic and European Contexts
svá ec ríst oc í rúnom fác,
at sá gengr gumi
oc mælir við mic.
I know a twelfth one if I see, up in a tree,
a dangling corpse in a noose,
I can so carve and colour the runes
that the man walks
and talks with me. 
To what belief system does this grisly presentation of necromancy—if, indeed, that is what it is—refer?  Does this “charm” (ljóð) project pure fantasy or might it reflect what was once an actual practice?
Hávamál 157 in its Nordic setting
And a few chapters later, in the enumeration of Óðinn’s magical abilities, the same text notes:
The story of Óðinn and Mímir’s head holds a unique place in the mythology,  insofar as in opposition to other prophetic “talking heads” in Old Norse literature (e.g., Eyrbyggja saga ch. 43), it is specifically Óðinn’s charm magic that allows or induces Mímir’s head to produce its utterances. 
þar er hann vissi vǫlo leiði;
nam hann vittugri valgaldr qveða,
unz nauðig reis, nás orð um qvað:(Baldrs draumar st. 4)
(Then Othin rode to the eastern door,
There, he knew well, was the wise-woman’s grave;
Magic he spoke and mighty charms,
Till spell-bound she rose, and in death she spoke:
(Larrington 2014: 235))
This scene, gruesome as the spectacle of human skulls nailed on tree-trunks must have been, lacks some of the eeriness of the scenario of revivified dead hinted at in Hávamál 157, but it does suggest a frightening range of post-proelial manipulations of the dead in Iron Age northern Europe. 
Figure 1. Detail from Lärbro Stora Hammars I. Photo by the author.
This “spirit” eventually foretells their fate, but most of all he curses in direst terms “the one who summoned me … dragged me from the infernal depths.” 
Figure 2. The one-eyed figure from the stave church at Hegge, Norway. Photo by John Erling Blad/Wikimedia Commons, CC BY-SA 3.0.
Hávamál 157 and the learned and ecclesiastical tradition
The same belief later appears in Peder Månsson’s Old Swedish translation of the late medieval Speculum lapidum of Camillus Leonardi, which similarly says that by taking Celonites and placing it under the tongue one can speak many prophecies about things that will come to pass.  Although we cannot know with certainty whether this tradition exercised influence on the way Óðinn’s powers are presented in our texts, it is likely that the existence of this parallel belief system would have been known among the clerics who possessed Latin learning and the other requirements necessary for engagement with natural magic.
a corpse swinging from a halter,
I cut and paint runes
in such wise that the man walks
and talks with me.
|Roðin es Geirvǫr
hon mun hylja
(Eyrbyggia saga ch. 43)
|Geirvǫr is bloodied
by the gore of men,
she will hide
(Eyrbyggia saga p. 116)