Note: This entry is vague on plot points to avoid spoilers. Those who have read the module should know the situation I am referring to.
I am running CoS for the first time. In our last session, the PCs obtained the Chapter 5 McGuffin, but awoke all the vampire spawn. Our next session will start with a chase – some of the PCs with the MacGuffin will be running back to where it needs to be returned. Others will be running back to the base of operation of the lycanthropes. The vampire spawn will pursue both groups, and thus some of them will likely arrive at the lycanthrope base of operations and be confronted by them.
I am a new-to-5E DM. I am trying to predict what the confrontation will play out like and to consider the best strategies for both parties. I would appreciate comments on whether my rules understanding and reasoning are sound.
The attacks of vampire spawn are claws (slashing or grapple, 1 or 2 attacks per turn) and bite (piercing plus necrotic; on an unwilling victim, the target must first be grappled to make an attack, 1 attack at most per turn). Of these, only the necrotic bite damage can harm the lycanthrope, since it has immunity to non-magical, non-silvered slashing and piercing. Thus the optimal strategy of the vampire spawn is to make claw attacks until one hits, choose to grapple rather than damage, and then attack with bite to cause necrotic damage. Once a spawn has established a grapple, it should cease making claw attacks and just make one bite per round (unless both hands may be used to establish separate grapples, see "here").
If the spawn starts its turn with no grapple in place, it has a 60% chance of a hit with its first attack, and will score on average (0.6)(7) = 4.2 with its bite, for an anticipated 2.52 points of necrotic damage. If it misses with the first claw, it has a 60% chance of a hit with its second claw, but then cannot bite that turn, and the lycanthrope then has a 50% chance of being able to break the grapple if it has a turn before the spawn’s next turn.
If the spawn starts its turn with a grapple in place, it has a 60% chance for a hit with its bite, for an average of 4.2 points of necrotic damage. Regardless of whether this attack hits or misses, there can be only one bite attack per turn (see "here" for whether it could benefit from a second grapple).
Thus, one on one, a vampire spawn is likely to take out a single lycanthrope in around ten rounds or so.
The lycanthrope has many options for forms and attacks, and all of them are capable of damaging the spawn except the curse of lycanthropy as the spawn is not a humanoid. Its optimal damage strategy is to use hybrid form for the increased movement and attack either with one sword and one missile, or with two swords. However, the average damage will be 5.5 per round, and the vampire spawn will be regenerating at least ten points per round and more if it hits with a bite. Furthermore, any time the lycanthrope starts its turn grappled it should spend its entire attack action to break the grapple, which will further reduce its damage output. Given this, in a one-on-one fight a lycanthrope is very unlikely to be able to take down a spawn, and even two or more lycanthropes on one spawn would be unlikely to win.
It seems like the lycanthrope’s best option is to assist the PC’s and encourage them to take out the spawn one at a time by concentrating attacks. If the lycanthropes cannot do this, they really should flee as they are clearly outmatched. If they can do this, possibly keeping themselves at a distance and attacking with missile fire might be appropriate, although it limits them to one attack per round. Alternatively, they could remain in melee and use the Help action to give PC’s advantage on attacks against the spawn.
Unbeknownst to the lycanthropes, they might receive assistance from a nearby NPC as well, although in the case of my party it is rather early for this reveal.
Finally, the lycanthropes have on hand several allied swarms, which is where I am most unsure of optimal action choices.
If the swarms start their turn with a lycanthrope grappled by a spawn but the lycanthrope going before the spawn, one swarm should ready an action to Help the lycanthrope on its turn, giving it advantage on escaping the grapple. Only one should do this since the lycanthrope cannot do better with having more Help on the same action.
If the swarms start their turn with a lycanthrope grappled but the the grappling spawn going before the lycanthrope (or the possibility that the spawn will go before the lycanthrope in the next round, if the swarm is the last of the three to go in the current round), at least one swarm should use its action to also attempt a grapple on the lycanthrope. The lycanthrope should choose to defend against the new grapple with Athletics rather than Acrobatics to give the swarm the best chance to succeed in the grapple (d20-2 > d20). If the grapple succeeds, the swarm should then move the lycanthrope out of reach of the spawn and thus end its grapple. If there are two or more swarms available per grappled lycanthrope, it is not clear to me whether one should Help the other to give it advantage on the attempt, or rather each one should try independently but successively.
If the swarms start their turn with no lycanthropes grappled, they should prepare readied actions to shove any lycanthropes who are grappled by a spawn out of the spawn’s reach as a reaction on the spawn’s turn, thus ending the spawn’s grapple before they have a chance to bite. Again, it is not clear to me whether multiple swarms should be attempting this independently or helping others that are. It would be less effective to shove the spawn (who have a better chance to resist) and it would be impossible to both grapple AND move the lycanthrope or the spawn as a readied reaction.
I would appreciate any comments on whether I have misunderstood the mechanics of this fight or that the overwhelming tactical goal of the spawn is to bite and that of the lycanthropes is to break spawn grapples before they can bite.