Here's what i remember, it's been years since I thought about these things, so some details might be slightly off.
Look at the weapon to see if it has any poison.
Touch the weapon to apply poison, if it holds any.
Pour the poison on the weapon to fill it, if the weapon is empty.
There are 4 or 5 different flare messages and only one of them does anything different; it will knock the target down. Otherwise all other flares just have a fancy looking flare messages.
The poison doesn't cause any initial damage, but it does apply a DOT that triggers after 30 seconds (yes, that is 30 seconds). You'll kill the target before the poison DOT ever triggers. So, sadly, these poison flares are pretty pointless if you ask me.
There are different levels of poison potency from weak to strong, they apply a few points of damage to upwards of 75 (if memory serves me right). The poison, per round (remember, it's 30 seconds for the poison to tick) dissipates by 1 point to the same amount the poison DOT is at (so if by some chance the poison hits for 50 points per round, it could dissipate 50 points after the first round).
There is no way to tell how much poison damage is applied when it flares.
The weapon will not flare on a target that already has an active poison flare against it.
If you want to enchant the weapon, you can, but ONLY if the weapon has no poison on it or stored in it. I never tried to ensorcell or add other services to it, but I'd guess the same rule would apply that the weapon would have to be empty of poison.
Lastly, even if the weapon is void of poison it will still give flare messages. The only useful thing about this is the flare that knocks the target down still knocks them down when there is no poison in the weapon.
I think that sums them up.
These weapons were released during the first run of the Dhu Gillywack, not sure about any other releases, but that is where I got my vultite falcata that I still have to this day that I enchanted to 6x