Thanks for making this a separate thread -- if someone tries to continue the rules discussion in the other thread from here on out, I'll redirect them to this thread.
Why would a vampire doing 1d8+4 damage prevent a giant elk hurting someone if it kneels on them. An unarmed strike does not specifically refer to claws. It can be anything as per crawford tweet.
Crawfords tweets are not official, but you are right that a monster can choose not to use one of its listed actions and can instead use an action defined in the Player's Handbook as being usable by anyone. The problem here is that Martial Arts isn't an ability usable by anyone -- only monks, and even then only in certain circumstances.
Half correct. It does not mention being unarmed. It only mentions unarmed strike. There is no such thing as "Monk unarmed strike"/"Regular unarmed strike", only damage that is dealt.
Re-read the Martial Arts ability: "You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon. This die changes as you gain monk levels, as shown in the Martial Arts column of the Monk table." (SRD 5.0, p.26)
There is only one unarmed strike ability, but if you have Martial Arts, you can use Dex instead of Str for the attack and damage rolls, as well as rolling different damage for the damage roll. You only gain those benefits "while you are unarmed or wielding
only monk weapons..." (ibid, emphasis mine) That is why being armed with a non-monk weapon is significant to this discussion.
There is no "house-rule" when there isn't a rule. The DM's are empowered to interpret gray areas. That hasn't changed. The difference is whether you believe this is gray or not. As always, expect table variation.
I've said myself that typically it falls to the DM to decide what a grey area actually is; others have accused me of saying that a DM could make a longsword do damage other than that listed in the PH if the DM decided a given situation was enough of a 'grey area' for her purpose. In response, Kalani and others have forcefully stated that, for the purpose of DMing an AL game, if something is written in the Player's Handbook, a DM must house-rule to provide a different ruling, and house-ruling isn't allowed in Adventurers League.
To my mind, the only 'grey area' here is whether a natural weapon can be treated as a simple weapon for the purpose of the Martial Arts rule. A DM who wants to say that it isn't can point to the weapon list and note that 'natural weapon' is not listed there under simple weapons, thus refuse to allow natural weapons to be treated as monk weapons. And as soon as a character is wielding *any* non-monk weapon, they lose the ability to use the benefits of Martial Arts. That interpretation would not be a house-rule and would thus be allowed in AL.
I'd say it's the recommended ruling as well, if I were still in the business of providing such things.
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Pauper