Will poison work on Polearm Master feat's bonus action (second)attack when using a poisoned halberd?
Poison can be applied only to slashing or piercing weapons, but it's not applied directly to the opposite end of the weapon. So, will it work?
The Polearm Master feat (PHB, p. 168) says, in part:
When you take the Attack action and attack with only a glaive, halberd,
quarterstaff or spear, you can use a bonus action to make a melee attack with the opposite end of the weapon. The weapon's damage die for this attack is a d4, and the attack deals bludgeoning damage. This attack uses the same ability modifier as the primary attack.
The description of the vial of basic poison says:
You can use the poison in this vial to coat one slashing or piercing weapon or up to three pieces of ammunition. Applying the poison takes an action. A creature hit by the poisoned weapon or ammunition must make a DC 10 Constitution saving throw or take 1d4 poison damage. Once applied, the poison retains potency for 1 minute before drying.
Will the second attack, if successful, apply the poison to the intended target?
Best Answer
As an edge case, the RAW technically allows this
The problem here is that this is an edge case. Normally, a slashing or piercing weapon is a weapon that does only slashing or piercing damage. Nowhere in the rules does it explicitly say that the poison must be delivered by those damage types. But since the rule only mentions the weapon and a halberd is a weapon that does slashing damage, technically the poison could be applied here even though the Polearm Master attack itself only does bludgeoning damage.
Logically, this shouldn't work
However, if we look at the rules from the perspective of what they are trying to say instead of the strict letter of the rules, I think we come to a much more reasonable ruling.
The rules state that a slashing or piercing weapon is what needed to deliver the poison. In the rules, damage types are almost always attached to weapon damage and not weapon identity. So, this wording is very unusual. Even so, the damage type is clearly a reference to the expected damage type of the weapon damage. Thus, I think it is highly reasonable to read the poison rules as saying "slashing or piercing damage from a weapon" because of this.
In fact, one of the sample poisons from the DMG (published after the PHB where the Basic Poison is written) is already written this way1:
Narratively this all makes sense as logical inference from the rules seems to be that the target must be cut or pierced in order for the poison to enter the body and take effect.
1 - Thanks @NautArch