From ErfWiki

Proposed Canon

Veil is a magic spell^[1] cast by a Foolamancer to hide units^[2] or cities^[3].

Limitations of the spell:

• In order to be able to veil units Foolamancer must be in the stack that is being veiled.
• The spell cannot be cast on the enemy's turn.
• Foolamancer must not be linked with other casters^[4]
• When veiling cities, only one city can be veiled at a time.^[5]

Spell name is Crypsis.^[6]

Speculation

Veil can blown if enemy warlords are present in the same hex as a veiled stack.^[7][8] The more units are in a veiled stack and the more enemy warlords are in the hex the higher are chances of blowing the veil.^[9][10] It is not clear whether regular units have less chances to blow a veil or whether they can blow a veil at all.

Apparently the veiled stack cannot be made completely invisible. Instead it appears to enemy units as something else.^[11]

Apparently veil is not always cast successfully.^[12][13]