At 3rd level, your otherworldly patron bestows a gift upon you for your loyal service. You gain one pact boon feature of your choice, which can be the following option.

Pact of Blood

When you begin to cast a warlock spell of 1st level or higher using your Pact Magic feature, you can choose to, as part of its casting, use your Pact of Blood to power the spell using your own life force. When you do, you don't spend a spell slot to cast the spell, instead spending a number of your Warlock hit dice equal to the spell slot level of your Pact Magic feature. For example, a 5th-level warlock's Pact Magic spell slots are of 3rd level, so they would spend 3 hit dice when they use this pact boon.

You must have a sufficient number of warlock hit dice remaining to spend in order to use the pact, and when you do, you also lose a number of hit points, as your own vitality is drained into the spell. The total number of hit points lost are equal to a roll of one of the spent warlock hit dice plus your warlock level. This hit point loss can't be reduced or negated in any way.

If the hit point loss from using the pact reduces you to 0 hit points, you don't fall unconscious until the end of the current turn or until you take damage, potentially allowing you to finish casting the spell. If this happens on your turn, you don't make your first death saving throw until the start of your next turn, though you still accumulate death save failures from taking further damage as normal.