The vervain plant.
Vervain is a scentless, flavorless herbal plant that is commonly used for folk medicines, but has value in many cultures - usually featured in supernatural and religious tales. Nonetheless, vervain is a primary means of protection against vampire compulsion, glamour, or mental mind control. It is also very harmful to a daeva vampire .
Vervain as a poison
Vervain can be put into a syringe and injected into a daeva vampire. It can also be put into a daeva's drink or within the blood of someone from which a vampire plans to feed. Once the effect of vervain takes place, almost
Damon Salvatore injected with vervain.
immediately after consumption or injection, the poison of the vervain spreads through the vampire's body in seconds, rendering him/her extremely weak - much like a sickness would cause severe fatigue and aching within the body. The vampire will not have enough energy to physically move and, if injected with enough vervain, will become unconscious.
It takes a little while for the vervain to pass through a vampire's system. Drinking blood significantly helps in speeding along that process.
Vervain as a weapon
Direct contact with vervain will burn a daeva serverly. Any weapon produced with vervain will deliver painful results - for example, a vervain hand grenade. Any items, such as chains or ropes, laced with vervain will subdue a vampire and cause him/her pain.
Vervain as protection
A human placing even a small amount of vervain on their person will prevent vampires from compelling or glamouring them or entering their dreams. Keeping vervain in their blood systems (for instance, through drinking vervain laced tea) will have the same effect.
Elena's vervain necklace
It takes 3 days for vervain to fully leave a human's system if they have been drinking it.
Vervain against Originals
Using vervain against an Original daeva produces half the affect it would a regular daeva. Direct contact with vervain will burn them but not significantly enough, for they heal too quickly. The same applies for vervain injections, they will slow down after injection but it will only take a moment for them to recover.
If a vampire drinks vervain and has it in their system, they can prevent themselves from being compelled by an original.
Stefan tricked into jumping into a vervain filled well
Immunity
A daeva vampire can never be fully immune to vervain. However, they can try to drink a little vervain every day in order to build some amount of resistance to it if injected. In this case, a double dose of vervain would be needed to produce significant affects on a vampire.