The Monster Manual answers this, explaining what you turn into based on where your soul goes. Being killed by a succubus doesn't change your destination, it's the fact that you have an evil soul that drags you down.
When the soul of an evil mortal sinks into the Nine Hells, it takes on
the physical form of a wretched lemure. (p67)
If you wish to become a low level devil or other being, you may need a promotion.
Archdevils and greater devils have the power to promote lemures to
lesser devils. Archdevils can promote lesser devils to greater devils,
and Asmodeus alone can promote a greater devil to archdevil status.
This diabolic promotion invokes a brief, painful transformation, with
the devil's memories passing intact from one form to the next.
So, simply be useful, and you may be promoted up from being a gooey, clay mess.
Alternatively, if you go to the abyss, you may be turned into a Mane.
Spawn of Chaos. The Abyss creates demons as extensions of itself,
spontaneously forming fiends out of filth and carnage. Some are unique
monstrosities, while others represent uniform strains virtually
identical to each other. Other demons (such as manes) are created from
mortal souls shunned or cursed by the gods, or which are otherwise
trapped in the Abyss. (p50)
Your form here is elevated by how much blood you spill, and whether others promote you.
If instead you go to Hades, you become a larva.
The layers of Hades are called the Three Glooms — places without joy,
hope, or passion. A gray land with an ashen sky, Hades is the
destination of many souls that are unclaimed by the gods of the Upper
Planes or the fiendish rulers of the Lower Planes. These souls become
larvae and spend eternity in this place that lacks a sun, a moon,
stars, or seasons. Leaching away color and emotion, this gloom is more
than most visitors can stand.(p63, DMG)
While in Acheron, you fight endlessly.
Acheron has four layers, each made of enormous iron cubes floating in
an airy void. Sometimes the cubes collide. Echoes of past collisions
linger throughout the plane, mingling with the sounds of armies
colliding. That's the nature of Acheron: strife and war, as the
spirits of fallen soldiers join in endless battle against orcs devoted
to Gruumsh, goblinoids loyal to Maglubiyet, and legions assembled by
other warmongering gods. (p66 DMG)