TA-418 is a chemical compound that played a large part in the Blackgaard saga.
TA-418 is a part of the Ruku Virus. It attaches itself to the virus making it adaptable to anything, where Ruku would otherwise die as soon as it was exposed to air.
The "T" stands for Tasha as in Tasha Forbes.
The "A" stands for Avery from John Avery Whittaker.