A Succubus can shapeshift into Small or Medium humanoid or back into its True Form form at will. Without wings, the Succubus loses its flying speed. Other than its size and speed, its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

With a kiss of her lips, an embrace in her arms, or something more intimate, a succubus could drain the life energy of a mortal who lacked stamina, weakening them in every aspect or wounding their psyche until they died or escaped and took time to recover. For mortals who were unwilling—and the certain few who were willing—the succubus would have to charm or grapple them to lay her kiss somewhere.  

Caught in the throes of passion, victims often don't notice their life being drained away, unless they keep a cool head. Moreover, the kiss of a demonic succubus could make the victim desire another kiss if they do not have the willpower to resist, and yet another, so they inevitably and willingly give up their energy to the succubus.

It is in  a Succubus' nature to charm its victims. You gain access to the Charm Person spell and always have it prepared. A traget charmed by you with this spell is charmed for 12 hours instead of only 1 hour.

If a creature charmed with this spell takes damage from your Succubus' Kiss the charm will not be broken.

You gain Proficiency in Persuasion and Deception. Additionally, if you roll on either of these to seduce another creature, you gain advantage on that roll.

Demonic Succubi thirst for life energy.

Each month a Demonic Succubus must consume life energy worth of four nights of passion. If the Demonic Succubus cannot do that they suffer a level of exhaustion. This level of exhaustion is not removed upon taking a Long Rest. The only way to remove that exhaustion is to consume the remaining life energy missing from the prior month.

While in their true forms, Demonic Succubi can use their claws to harm their foes.

You gain Darkvision in a range of 60 ft.