Necessary objects for the ritual:
-Warding Candles (x4)
-Spell: Projectile||Fire Damage
First, place an obelisk in the middle of a small, clear area. No power is required to be generated by the obelisk for this ritual. You are going to draw an specific shape around the Obelisk, with Wizard's Chalk, and place Warding Candles in the inside corners. See the bottom right of this post for a diagram of the setup.
-NOTE: in this diagram, wizard's chalk is placed on coal blocks, and warding candles placed on Lapis Lazuli blocks. This is merely to help a person see where to put the items, and ARE NOT REQUIRED FOR THE RITUAL.
-Note that these Warding Candles must NOT be attuned to a specific block. If you don't know the specifics of Warding Candles, don't worry about it- Warding candles are crafted automatically non-attuned.
If your setup looks like this, you are ready for the next step. In your inventory, you should have a Monster Focus and a Sunstone. Throw these within the shape of the wizard's chalk.
After this, simply cast your Projectile | Fire Damage spell directly at the obelisk, and voila, A ritual of corruption has been performed! You'll know you did it right when all of the objects in your ritual disappear, aside from the obelisk, which now appears to be some sort of dark vortex.
Also, do this somewhere you find reasonable, because you cannot retrieve the dark vortex if you break it, only a obelisk.