[Saulot]

Entire post corrected for clarity/completeness so that there is one complete explanation on just one post:

According to page 80 of the manual, a squad takes a morale check when:
"The squad has been subjected to a spell causing Fear (e.g. Terror)"

So, to clear this up, there are two situations. Fear the unit ability, and Fear from spells.

(A) Unit Ability Fear works as such:
The fear targets X squares, where X equals Fear strength + 6. The morale check the creature takes is the difficulty of 10 + (Fear/5 rounded down).

Ex. A POD which has Fear +19 (Basic Fear +10, plus an additional +9 from 9 levels of death), would have a fear aura that covers 25 squares, and makes creatures take morale checks at 13.

(B) Fear Spells:
Fear spells work the same way as the fear unit ability but they only cover an area as specified per spell.

Ex. So, a Fear +10 attack (such as the one from the terror spell) requires them to pass a morale check against 12.

(C) Bonus: Both fear effects (including spells such as terror) lower morale of the units under the fear effects (though I'm unfamiliar with the formula, perhaps a dev would chime in?)