This mod is given at the penultimate mission of The Sacrifice, automatically ranked to level 5.
Sacrificial Pressure is essentially the Pressure Point mod with Legendary rarity and 5 more ranks. It is considered the same mod as its Common counterpart, and thus both mods cannot be equipped together. The same is true for Primed Pressure Point.
The set bonus for Sacrificial Pressure increases all stats given by the mod by 25% of mod's value per set pieces. At max rank with full set bonus, Sacrificial Pressure grants a melee damage bonus of +137.5% and a damage bonus of +41.3% against Sentients.
This mod only surpasses Pressure Point at rank 9 with both set mods but fails to surpass Primed Pressure Point even at rank 10.
Umbra mods have a unique polarity, whose matching polarity can currently only be found on Excalibur Umbra, Exalted Umbra Blade, and the Skiajati. It is not possible to (re-)add an polarity to a mod slot, so it's advised not to overwrite a pre-existing one.
Similar to other faction damage mods (i.e. Smite Grineer), the bonus damage against Sentients gets applied on top of the base damage and any added elemental damage of the equipped melee weapon (see Damage for complete mechanics).
When equipped alongside Sacrificial Steel on Exalted Weapons, both mods will not show any set bonus, but set bonus effects will still be applied to the final stats.