Summon Cultist is a Path of Exile skill. At 75/50/25% Life, summons an add that will tether to Medved. Voiceline “Aid me”.
Medved, Feller of Heroes, the expedition boss, uses this skill.
At 75, 50 and 25% health, Medved will summon a Cultist to aid in the fight. This conditional needs you to have a build capable of ignoring the Cultists while damaging Medved in order to get him to summon the Cultists. Wait until all 3 have been summoned and are low before you decide to burst them together.
As stated previously, Medved will summon Cultists at 75, 50 and 25% health. This conditional just means you need to be able to kill Medved while ensuring you stay away from the summoned Cultists and not damage / kill them.