Underworld Seeds

It's says it does 15 Damage but no damage gets registered when it lands.

My guess would be that it does 15 damage each turn.

No it does not.

underworld seeds saps health every time you do damage, 15 power is how much damage it does in addition to the attack you did that then determines how much health you get back

It's a much weaker version of underworld plant. 15 power doesn't mean 15 damage. It depends on your matk and opponent mdef and heals for only a fraction of he damage you do...

Or does it heal as much as it damages? I just assumed was less but not sure...