This functions continuously and does not require a Psychic test to use. Any enemy unit that wishes to assault a creature with this power must take a standard Morale check. If the check is failed, the unit quails before the horrifying psychic presence of the Tyranid hive mind and may not make an assault that turn.
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): The Horror only applies to enemy units charging the Hive Tyrant model itself, not to units charging a different model in a unit the Hive Tyrant has joined.