You can draw on your profound relationship with your patron to beseech their power. As an action, you can reduce your hit points by half your maximum, spilling your blood to bring an avatar of your patron to your side.

The avatar fills a 10-foot cube in a space you can see within
30 feet of you, it remains within 30 feet of you, moving with
you, it is immune to all damage and conditions, and it re-
mains for 10 minutes or until you are reduced to 0 hit points
or you dismiss it (no action required). While the avatar remains you gain the following benefits:

• If your total for a Charisma check is less than 5 + your Charisma score, you can cause the roll's result to
  become 5 + that score.
• You have advantage on all saving throws and
  resistance to all damage.
• Your spells can originate from the avatar's area, instead of from you, and you can add an extra 2d10 damage to one roll of a spell cast from the area.
• As a bonus action, you can choose a creature that can see the avatar and force it to make a Wisdom saving throw against your spell save DC or become frightened or charmed (your choice) until your patron vanishes.
• As an action, you can ask your patron a question, as if by the contact other plane spell, and your patron must answer truthfully, to the best of its ability.

Once you use this feature, you can't do so again for 7 days, or until you use your 9th level Mystic Arcanum to do so again.