The Serpent

Be it through a powerful yuan-ti, naga, or other snake-like entity, you have made a pact with a serpentine creature to gain  magical abilities.

Bonus Cantrip

At 1st level, you learn the poison spray cantrip.  It counts as a warlock cantrip for you, but it does not count against your number of cantrips known.

Serpent's Fangs

At 1st level, you gain the ability to add an extra bite to your spells.  When you hit a creature with a spell attack, you may an additional 2d4 to the damage rolls for that attack.  This extra damage increases to 2d6 at 5th level, 2d8 at 10th level, and 2d10 at 15th level.  You may do this a number of times per day equal to your Charisma modifier.  You regain these uses when you finish a long rest.

Slither Away

Starting at 6th level, your link to the Serpent allows you to magically escape when needed.  When you are reduced to 0 hit points, you may magically polymorph into a constrictor snake for 1 minute.  Once you use this feature, you can't use it again until you finish a long rest.

Flexible Anatomy

Starting at 10th level, your body takes on reptilian traits, like the ability to coat your skin in slime or your bones becoming more pliable.  As a bonus action, you may coat yourself in an acidic slime until the start of your next turn.  While coated, you may free yourself from a grapple or restraint and any creature that touches you or hits you with a melee attack while within 5 feet of you takes 5 (1d10) acid damage. Once you use this feature, you can't use it again until you finish a long rest.

Summon the Snakes

Starting at 14th level, your patron gives you the ability to transform sticks into snakes.  You may transform a pile of sticks, arrows, or similar-sized pieces of wood into a swarm of poisonous snakes, or you may turn a quarterstaff or similar polearm into a giant constrictor snake.  This creature acts as your ally and obeys your spoken commands.  This creature remains for 1 minute, after which it turns back into the original materials.  Once you use this feature, you can't use it again until you finish a short or long rest.