Tengu format ruling
Venom Swamp

Card effect (current official text)During each player's End Phase: Place 1 Venom Counter on each face-up monster, except "Venom" monsters. Monsters lose 500 ATK for each Venom Counter on them. Destroy a monster if its ATK becomes 0 by this effect.
Rulings
- This card’s effect to place a Venom Counter on each face-up monster starts a Chain.
- This card’s effect to destroy a monster whose ATK is reduced to 0 does not start a Chain.
- If a monster’s original ATK is 0, or its ATK has been reduced to 0 by another card effect(s) and a Venom Counter is placed on it, it will not be destroyed by "Venom Swamp".
- If Venom Swamp is negated by Imperial Order, etc., Venom Counters placed by its effect remain in play.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Tengu forum — sourced change requests get reviewed.