Expanded Spell List

Your Symbiotic Patron lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Symbiotic Patron Expanded Spells

Spell Level	Spells
1st	Detect Magic, Disguise Self
2nd	Enhance Ability, See Invisibility
3rd	Nondetection, Sending
4th	Fabricate, Polymorph
5th	Wall of Force, Hold Monster

Symbiotic casting

At 1st level your symbiot grants you one cantrip when you bond with it. When you gain this feature, choose one cantrip from any class’s spell list.  This cantrip doesn't count against your number of cantrips known. If it doesn't appear on the warlock spell list, it is nonetheless a warlock spell for you.

Symbiotic Assistance

Starting at 6th level, the warlock has flashes of knowledge in topics previously not studied that come from the Symbiot.  You add half your proficiency bonus, rounded down, to any ability check you make that doesn’t already include your proficiency bonus.

Reinforced Will

Starting at 10th level, your Symbiot helps to shores up your mental faculties and endurance, helping to sustain your concentration on your spells when needed most.  You now have advantage on Constitution saving throws made to maintain concentration on spells. 

Starting 14th level, this ability evolves so that you use your Charisma score modifier in place of your Constitution score modifier for concentration checks.

Of Two Minds

Starting at 14th level, the connection with your Patron has evolved to a point that your thoughts and that of your Symbiot's cannot be told apart.  Because of this connection the Symboit gains the ability to hold concentration for any spell cast by its host.  This allows the Warlock to cast and hold concentration for a second spell.

Due to the amount of focus needed to maintain concentration on two spells prevents you from casting another spell. Example: You cast and are holding concentration on the spells Darkness and Fly.  When you cast any other spell, weather it requires concentration or not, you must loose concentration on one of the two spells active do so.

You can choose, upon casting, who will hold the concentration. 

If you are maintaining the concentration on the Spell, as an action you can choose to have your Symbiot take and hold the concentration for it.

Your Symbiot receives the benefits of Reinforced Will using your Constitution saving throws to maintain concentration on spells.

Once you use this feature, you can’t use it again until you finish a short or long rest.

that is pretty cool flavor wise, i've never been good at balance w/o playtesting it, so hopefully someone else can help you with that.  butt it's a really great concept