This is called Sita's fruit in India - Sita was the wife of Rama, the hero of the epic Ramayana - and custard apple elsewhere.
The fruit is soft, grainy off-white, and wrapped around glossy black seeds. The best way to eat it is to cut of the top and spoon out the fruit. It has a delicate flavour that I like - not everyone does.
Pencil and watercolour wash, with a bit of ink here and there in the shadows.