Works best on life stacks because Life + Sigil + Dark Vial Heal + healing Pot = wow, that really is alot of life to cut through to kick him out of a lane.

It's a better choice for Rook than others because his HPs are high, even without BoTF.

I cannot think of any other high HP character neeeding it because they all have idols, or have amazing combo results with Blade of the Serpent.

Though, now that I'm thinking about it, it might be a pain on the moo moo rocket too.... eg: idol heal + high HPs + sigil/vial = insane life to cut through. Haven't tried, so that is just speculation on my part.

If you pop a sigil then use it, they have to interrupt it, then you can teleport away uninterrupted. Or, they can let you use it can you will destroy them with +50% to your health, then + 33% of the new total. 

6000hp + sigil = 9000hp 

vial heals 3k, or 1 half of your normal health in 1 second

That is to say, if you get down to 500 health (out of 6k), use a sigil and the vial you go from 500 to 6500 in 1.2 seconds

Not to mention you can do this every 45 seconds. It's very good, don't know if it's better than BoTF though.

It could be viable with a sedna in the late game if the sedna gets the cd reduction skill.