The term tick is a misnomer, as this creature is not an arachnid, and doesn't possess a mouth, as such. It's around 3", symetrical, and generally spherical in shape, with six legs ending in viscious looking prongs, and twelve eyes in three clumps around it's body. The prongs are the Tick's mouths, from which it excretes its digestive poisons and sucks up the slurry that is produced.
The poison is both highly necrotic and an extremely effective aensthesia, making it a popular choice of those with less than firm morals that are willing to risk their lives trying to milk one of these.
A single tick is no threat, they're not fast and can easily be crushed under the sole of your boot; the real threat is when they breed, or are used as a weapon. A single demon can be infested with as many as half a dozen of these, not a problem for the demon which is largely resistant to the ticks' poison, but when that same demon is summoned to battle by some foul mage, and it falls, the ticks will first lay their eggs in its corpse, and then move on to the other corpses, resulting a veritable apocolypse when all the eggs hatch. There are tales of small kingdoms being buried underneath a swarm of these things.
Native to Hell, this creature preys upon the demons that were banished here, a torment for the tormentors. They thrived just as well as everything else in that blighted realm, which is to say barely. They were just big enough to be able to prey upon the demons, but not big enough to attract their attention; it wasn't until they were introduced to the mortal realm that they really became a scourge. With no natural predators, and with the soft flesh of mortals to feast on, the Demon Tick became to the mortal realm what kudzu became to the Southeastern US.
Is not disease the rule of existence? There is not a lily pad floating on the river but has been riddled by insects? -Henry David Thoreau