The Nerazim believe that their connection to the Void strengthens near death. To this end they have employed immortal technology on some of their warriors to hold them in such a state of near-death indefinitely, greatly boosting their powers.
The annihilator has the same anti-armor attack as the normal immortal, but instead of the immortal's Barrier, the annihilator is armed with a Shadow Cannon that can attack ground or air units. The cannon deals high damage to enemy units, but has a substantial cooldown. This effectively makes the annihilator a much more offensive-based unit to the immortal's defensive durability.
When the player later acquires the Tal'darimvanguard, the vanguard outclasses the annihilator as a dedicated anti-armor unit due to its splash damage and stronger attack. However, the annihilator's shadow cannon remains effective in missions where the player faces powerful enemy units, as shadow cannon can rapidly deal more damage to single targets than the vanguard's normal attacks. The annihilator is also able to use shadow cannon on air units, helping to alleviate its vulnerability to airborne foes.