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Tossing a synth relay grenade causes a level 5 generation 1 synth to spawn where it lands, and serve as reinforcement.
- Allies of the Institute can purchase up to 30 synth relay grenades at a time from the synth requisition officer after the main quest has been completed.
- Eight grenades are found lying on a table in the small weapons testing area of the Institute Advanced Systems area.
- Five grenades can be found on a shelf in the Institute Synth Retention Bureau.
- Three grenades can be obtained via a Speech challenge from the Courser during the Institute version of the quest The Battle of Bunker Hill.
- Can be found by pickpocketing an Institute Courser's main weapon from their inventory, exiting, then checking again. They'll equip the grenade, and it will become visible for stealing.
- Can be found as loot on Coursers at higher levels.
- There's no limit to how many synths can be teleported in, and is only constrained by the number of grenades you have, making the relay grenade potentially very effective at overwhelming an enemy position with a great number of expendable allies.
- If thrown on an NPC, the NPC affected will turn hostile. This is due to the teleportation effect created by the grenade causing approximately 1 damage in a small radius.
- Spawned synths will be hostile towards settlers and dismissed companions, regardless of faction alignment.
- If one still has grenades after destroying the Institute and uses them, the synths will be hostile towards the player.
- If a settler or provisioner equips a relay grenade and throws it, the synth will still attack them, and the grenade will disappear from their inventory (unlike other thrown explosives equipped by NPCs).
- Using unarmed in V.A.T.S. on a spawned synth, especially with the Blitz perk, the Survivor can reach otherwise unreachable areas in the game. The move can be cancelled after teleporting and before hitting the synth.
- Though synths typically spawn with Institute weapons and fusion cells, they sometimes spawn with special, more valuable weapons in their inventory, making it worth killing the spawned synth once it has outlived its usefulness and looting these items.
- Killing enough friendly synths from the relay grenade will eventually make all synths that are not at the Institute hostile toward the Sole Survivor.
- Synths spawned by these grenades will disappear if the Survivor leaves their cell for a few days.