I am DMing an Artificer for the first time and we have some questions about how his homunculus servant works. This question is about spells cast by the artificer but delivered by the servant.
The homunculus servant has, as a reaction, the following ability:
Channel Magic. The homunculus delivers a spell [the Artificer casts] that has a range of touch. The homunculus must be within 120 feet of [the Artificer].
The homunculus servant moves on its own turn:
In combat, the homunculus shares [the Artificer's] initiative count, but it takes its turn immediately after [the Artificer]. It can move and use its reaction on its own…
If the homunculus was already within touch range on the Artificer's turn, this would be simple: the Artificer casts the spell and the in-range homunculus immediately reacts to deliver it by touch.
However, this would require the homunculus to already be in position, and everything about their build suggests that is a bad idea; they have low AC, low hp, a flying speed, evasion ability, can take Dodge uninstructed, and their only natural attack is ranged. It looks like the homunculus is designed to be a ranged support creature, designed to move in, deliver spells, and move out.
I am having difficulty understanding how this is supposed to work in practice within the timing and action economy rules of combat.
It could be that since the homunculi move on their own turn, after the Artificer's turn, once they are moved into touch range of their target they would then have to wait an entire combat round with every other creature moving and acting before the Artificer could later cast the spell that the homunculus moved within range to deliver. During that round, of course, their target could move (rendering them again out of range) or they can be attacked and incapacitated. This seems like it would severely reduce the utility of the homunculus as a vehicle for delivering spells.
It could be that the homunculus is meant to deliver its Artificer's spell on its own turn. In this case the homunculus would start the Artificer's turn out of range of the target, the Artificer would cast their spell, and then the homunculus would move into range to deliver the spell as a reaction on their own turn, with the reaction trigger being "when I get within touch range of the target". They could move their 30 feet flying speed, or as a bonus action on the Artificer's turn, the Artificer could command them to Dash, allowing them a 60 foot move. This seems odd in that the homunculus is then using a reaction on its own turn (legal but typically rare), and it also begs the question of what is the status of the Artificer's spell in the time between when it is cast on the Artificer's turn and when it is delivered on the homunculus' turn. Is it like a Readied spell, where the Artificer has to maintain concentration? What happens to it if the homunculus never arrives within target range on its turn?
Finally, it could be (as Medix2's links in the comments might suggest) that the Artificer casts the spell as a Readied action on their turn, holds it until the Homunculus' turn, and releases it as a reaction when the homunculus is in range, and then the homunculus reacts to the reaction of spell release by using their Channel Magic ability. This does have the advantage of using the homunculus the way its build implies, and having defined mechanics for the Artificer's spell (those of a standard Readied spell). But it seems like a rather inelegant approach since it requires an action and two reactions simply to deliver a single spell, requires the homunculus to use its reaction on its own turn, and requires it to react to another reaction. I would also argue that the Chanel Magic ability, rather than saying "delivers a spell you cast" should say "delivers a readied spell you have cast" to better reflect the correct timing.
Does the homunculus have to deliver spells on the Artificer's turn, or can the spell be held until the homunculus' turn? If it can be held, is it by means of a standard Readied Action from the Artificer, or does the Artificer-homunculus combination itself permit the holding?
Best Answer
The readied spell interpretation seems to be the most plausible interpretation in my opinion. It does require concentration, and the spell slot would be lost if the condition isn't met (just like with a usual readied spell), however, since the homunculus takes its turn immediately after yours, the odds of that concentration being broken is pretty low, as it would require either a readied action from your opponent whose trigger is your spell or a poorly-timed legendary action from a DM specifically looking to screw you.
Now, the real question is why would the homunculus work that way, and not have a specific rule dictating that you can cast spells on its turn instead, and my thinking is that there is a scenario where that could be useful, which is when a creature moved into touch range with it before your turn. The homunculus could then immediately use its reaction to cast your spell. It could then on its turn move to another target or simply away from harm.
TL;DR: The readied action interpretation (i.e. you ready a spell and set the trigger as "when the homunculus is within range) seems to be the most plausible to me. You can also use the homunculus's reaction on your turn to cast the spell without readying it if it is already within range.