How would a party of three level 10 characters fare against a CR 12 archmage?

Question

I am planning a level 10 dungeon crawl where I would like the final encounter to be an archmage. At the moment my party consists of a bard, a ranger, and a fighter with the knight subclass from UA (all level 10). I want this encounter to be difficult, if not extremely so, yet not impossible. According to XP calculators and such, this encounter is deadly however it is still within the "max xp" for the party.

Because I am not sure exactly how this will go and don't want either the party or the mage to be killed in the first couple rounds, I am having this encounter in a place where things could be added or subtracted from the fight rather easily. It is happening in a large throne room-type space and I figure that I should be able to feel out the battle after the first round and add other enemies if I need to. I also could simply choose not to use the wizard's high level spells in order to nerf the encounter on the fly.

Is the CR 12 archmage still too powerful for this party?

Answer 1

I have run this encounter. They will win easily.

I have run a very similar encounter, with 4 PCs at level 10 against a single Archmage, with all of her spells pre-cast on herself, as the first part of a multi-stage boss fight. As you describe, the battle with the archmage took place inside what was basically a large empty building. The players easily blew through this stage of the fight--even with Time Stop to set up defenses, the Archmage was soundly defeated without a single PC going down.

First, consider that while the Archmage has access to 9th level spells, all of her high-level spells are nondamaging. The strongest damage-dealing spell she has is only 5th level. While Time Stop is potentially useful, it's mostly good for letting her run away with Teleport. Additionally, she only has 99 HP, which means that if she does get caught in a focus fire from PCs, she's going down very quickly.

Second, the DMG has some guidelines for how difficult an encounter is, based on XP thresholds. An Archmage yields 8400 XP, and DMG 82 states that for 3 level 10 characters, the XP threshold for a "deadly" encounter is 8400 XP. That means the Archmage is barely on the edge of being "deadly," which in my experience usually means "one or two PCs actually take some damage". In fact, the daily adventure XP budget for 3 level 10 characters is 27,000 XP, or basically three Archmage encounters before a long rest.

Finally, a single-monster encounter is always on the losing side of the action economy, which further reduces the difficulty of the encounter. For example, if your bard manages to get a Hold Person off on the Archmage, the encounter is basically over, since the PCs get two consecutive turns against the Archmage.

How do you make it more difficult?

You should really be asking, "how do I make this encounter more difficult?". In my case, I made the archmage a multi-stage boss, as mentioned above. In the second stage, I gave her significantly improved survivability, by giving her a regenerating barrier with high HP, which forced the PCs to focus down a portion of the barrier before they could even touch the Archmage. Moreover, I gave her a number of minions under her control, to even out the action economy a bit more.

Overall, these changes did not make the encounter that much more deadly for the PCs, but it did force them to be more creative in their abilities and tactics than a straight fight in an empty room.

Answer 2

This is extremely situational.

In a neutral arena, level ground, encounter without forethought, the party of 3 10th levels will most likely defeat the 18th level 12CR archmage pretty quickly, if they have decent tactics and mobility. Numbers matter a lot in this game, due to the action economy.

At the other end of the spectrum, if the archmage is in his own home space, and has had time to set up Guards and Wards, various Glyphs of Warding, minions and/or summoned creatures, a Contingency, a Teleportation Circle, choose his prepared spells accordingly, and various other preparations, the party would have no chance.

This really depends on you, as the DM, and how you write the encounter and the preparations for it.

Answer 3

Considering that CR is just a guideline, it could result in a Deadly encounter for your party or an easy one as well. Their chances of succeed depends on several other conditions not considered by the CR calculation or definition, such as terrain obstacles, previous encounters, the current party resources at the final battle, active magic effects, and so on.

So we need another indicator.

Taking a look at the "Creating a Monster" section in the DMG, there are some guidelines there for determining the CR of a custom monster. Specifically on page 274, they proposed a Monster Statistics by Challenge Rating table. It displays several indicators for determining CR, one of which is Damage per Round.

This considers the monster's damage output per round, which is the average result of the damage rolls the monster can get every round.

So we can borrow this idea and make some calculations on what could the Archmage's average damage output per round in order to get an idea on how deadly or easy would be for the party.

Doing some math, here is the average and maximum damage for each of the most damaging offensive spells the Archmage has (provided in the link) considering higher level cast (level 9, if possible):

• Cone of Cold: Average is 54 and maximum is 96 cold damage per affected party member, if unsuccessful CON saving throw (average of a d8 is 4.5, times 12)
• Lightning Bolt: Average is 49 and maximum is 84 lightning damage per affected party member , if unsuccessful DEX saving throw (average of a d6 is 3.5, times 14)
• Magic Missile: Average is 39 and maximum is 55 force damage (average of 1d4+1 is 3.5 per dart, times 11 darts, rounded up)

The rest of the spells are mostly defensive or non-combat ones.

Moreover, you could also calculate the average damage the players can inflict and the average amount of hit points they can regain, per round, to compare.

With this in mind, you can get a pretty good idea on how your party will fare in this battle and adjust accordingly.

If you think your players can comfortably resist the calculated damage, then they will most likely fare well, even find the encounter easy.

The downside, it can take a lot of time to calculate everything, it's almost a simulation, but it's worth it if you want your party to have a challenging encounter.

Finally, as per my personal experience, I've done these calculations and the necessary adjustments before with a boss on a final battle . As result, the party had a hard time, many characters went unconscious during the encounter, but they managed to beat the boss after many rounds of intense combat.

In this case I also considered the party's and the boss and its minions' strengths and weaknesses, including on who to target specific spells knowing the corresponding attributes needed for the spell saving throws. I hope it helps you.