Randomly, at later waves, if you didn't get any probes to help protect you or if you reach the queens wave... There's a 50% chance between waves during the day, the Protoss fleet will "appear".
If you have a rescue beacon, you can now try to initiate communication with them to help you by destroying the zerg forces. Note that the fleet isn't that powerful so at most they can send in troops to designated locations that you tell them.
How to do this:
Once Protoss appears, use your rescue beacon and use the ability "Communicate" with them, then depending on how you play through the game...
If you managed to get a big enough base consisting of 12+ supply within a 50 range radius (meaning loads of structures in one place)
+
If you managed to get a probe and build defenses with them.
Results:
The rescue beacon will change it's button, resulting in you being able to designate a map area for the protoss army to attack.
IF you lack both points.
AND you didn't try to communicate with them...
The protoss army will plan to use a Purifier and a countdown will start. The purifier will spawn and fly to the designated location, and start blasting a hole into the planet.
You have two choices at this point, destroy the purifier, or escape via transport ship.
Destroying the purifier angers the protoss, results in them spawning small attack fleets that will attack both zerg and terran alike, including your protoss structures.
It is possible for the zerg to destroy the purifier, since the countdown timer works throughout the waves. The purifier will be highly shielded though, so it'll be hard.
Also note, the beam from the purifier will do damage in a large AOE (not TOO high, but a constant damage per second), so the only units capable of destroying are nukes or special units like ravens or any other units like Big Bertha.
Another idea is Raynor's Hyperion could be passing by, and you need to use a rescue beacon to initiate a rescue force by them, it would result in a smaller but elite force but essentially they help you fight back the zerg (of which sets your objectives to destroy all zergs).
If you have a rescue beacon, you can now try to initiate communication with them to help you by destroying the zerg forces. Note that the fleet isn't that powerful so at most they can send in troops to designated locations that you tell them.
How to do this:
Once Protoss appears, use your rescue beacon and use the ability "Communicate" with them, then depending on how you play through the game...
If you managed to get a big enough base consisting of 12+ supply within a 50 range radius (meaning loads of structures in one place)
+
If you managed to get a probe and build defenses with them.
Results:
The rescue beacon will change it's button, resulting in you being able to designate a map area for the protoss army to attack.
IF you lack both points.
AND you didn't try to communicate with them...
The protoss army will plan to use a Purifier and a countdown will start. The purifier will spawn and fly to the designated location, and start blasting a hole into the planet.
You have two choices at this point, destroy the purifier, or escape via transport ship.
Destroying the purifier angers the protoss, results in them spawning small attack fleets that will attack both zerg and terran alike, including your protoss structures.
It is possible for the zerg to destroy the purifier, since the countdown timer works throughout the waves. The purifier will be highly shielded though, so it'll be hard.
Also note, the beam from the purifier will do damage in a large AOE (not TOO high, but a constant damage per second), so the only units capable of destroying are nukes or special units like ravens or any other units like Big Bertha.
Another idea is Raynor's Hyperion could be passing by, and you need to use a rescue beacon to initiate a rescue force by them, it would result in a smaller but elite force but essentially they help you fight back the zerg (of which sets your objectives to destroy all zergs).
