The description of the Mace of Disruption states:
When you hit a fiend or an undead with this magic weapon [...] If the target has 25 hit points or fewer after taking this damage, it must succeed on a DC 15 Wisdom saving throw or be destroyed. On a successful save, the creature becomes frightened of you until the end of your next turn.
Some undead are immune to the Frightened condition. Does the Mace override that immunity? Or does it only cause Frightened in those undead which are not immune to the condition?