Touch of Terana

You create a ghostly hand with splotches of dark blue on the skin of the hand. The hand appears in the space of a creature within range. Make a ranged spell attack against the creature to assail it with the touch of Terana. On a hit, the target takes 1d8 necrotic damage and can't regain hit points until the start of your next turn.
After the spell is cast, the creature takes an additional 1d4 necrotic damage at the start of each turn, this lasts until the spell either ends or is dispelled.
The spell ends on a successful Constitution saving throw which the target can make at the end of each of its turns. 

Once cast a dark blue, mold-like, fungus, with a paler blue margin begins growing on the surface of the creature's skin, the growth focusing on the area of where the hand made contact. The fungus grows each round that the spell is active, covering a larger area of skin. While the spell is active the fungus-covered skin has a velvety or waxy texture. However, once the spell ends the fungus-affected area begins to dry out, its texture slowly turning to a rough, crust-like one. If the target is not healed within 5 rounds of the spell ending the fungus will leave a gruesome scar. This scar is slightly blue in hue and has a dry, rough texture, similar to that of dead bark. 

If you hit an undead creature, it has disadvantage on attack rolls against you until the end of your next turn.

This spell's damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8). 

The visual description of the fungus is based on the crust fungus; Terana Caerulea, commonly known as the Cobalt Crust Fungus. 
For some more visual references, look up 'Terana', 'Terana Caerulea' or 'Cobalt Crust'.