Soldier's Resistance is a survival-scaling mutation that increases the player's maximum health while it is equipped.
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Increases the player's current maximum health by a percentage value, meaning the extra max health gained from Soldier's Resistance increases each time their max health is increased through other means.
However, Soldier's Resistance is additive with any non-stat-based max health increasing bonuses, such as that from Dead Inside.
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When paired with Necromancy, Soldier's Resistance actually allows one to get a slightly higher amount of extra health before the healing can no longer occur. This is because their maximum health pool has increased, so the amount of extra healing they can achieve increases with this as well.