I answered my own question: it summons about a dozen low level units near the caster. Doesn't seem very useful considering the mana cost/time to cast.

It summons a random selection of Highman Units, whether a large number of low-level troops or a pair of high tier units, or an Astra. However only those which are "Highman Racial" (so not Astras or Valkyries, which are merely nonracial units trainable from Highman towns) can be recruited with gold.

Also I think if you recruit them with gold they still disappear after a few turns, but that can still be a useful option.

In vanilla the spell's extremely overpriced, but that's easily fixed in a mod