Alberta's Split-Self

Invented by a undead elven wizard to assist in high magic you draw upon your own essence to form temporary copies of yourself from the weave. To do so as a mortal is madness and bound to lead to a painful death.

As a action you unravel your own life essence drawing upon the weave to create at least 1 temporary copy of yourself who is identical to you in every way. Each copy shares your soul and draws from the same pool of  HP, spell slots and any equipment and and benefits/negatives you may be under.
You are telepathically linked to the copies and can chose for them to act together to cast spells at the same time otherwise they will take their turn immediately after yours however they can only use their action to dash, cast a spell, use a object, dodge, help or make 1 attack.
You can have up to your MAX HP number of copies active at any one time and increase/reduce the number of copies as a bonus action as long as the spell is in effect.

While this spell is in effect you suffer exhaustion at the beginning of each of your turns as you are effectively fueling the spell with your life energy. You take 1 point of exhaustion each round per copy with the effect doubling until you dispel the copies or drop to 0 hp. (eg 1 level of exhaustion after 1 round, 2 added at round 2, 4 added at round 3, and 8 added at round 4...assuming your not dead by this point! ).

When you end the spell you can chose which of the copies remains however if the spell is ended unwillingly you dropping to 0 or being dispelled by another this is determined at random.

Note that you and the copies can both can concentrate on different spells however you pay for all the spell costs and share the same health pool and effects - this means that if both you and the copy cast a 2nd level spell you must pay two 2nd level spell slots and if you and the copy both are take 5 points of damage from being hit by a area of effect spell (eg Fireball) your total HP is reduced by 10. In terms of targets any spell or ability that takes one of you targets all your copies and any effects are shared by both yourself and the copies. This means that unlike area of effects any direct damage/healing and effects are shared - eg if one of you is hit for a Branding Smite doing 5 points of damage the damage is still only 5 - however you and all copies fall under it's effect to shed light and are unable to gain the benefits of invisibility!

Updated from Playtesting for more fun for casters, and clearer rules/easier to manage for DM's.