How can I handle a task that combines two skills?

Question

I'm the dungeon master and my party is about to encounter some Helmed Horrors. They are inanimate at first, giving the adventurers the chance to check them out.

Helmed Horrors have a special ability that reads:

Spell Immunity. The helmed horror is immune to three spells chosen by its creator. Typical immunities include fireball, heat metal, and lightning bolt.

I want to give my adventurers the chance to find out which spells the Helmed Horrors are immune to, prior to engaging them. I could just use either the Investigation or the Arcana skill for this, but I would prefer if it were a combination of the two.

How can I combine both skills such that both skills are relevant? More specifically, I want a character unskilled in either skill to have significantly lower odds of getting good results. In other words, both skills should be required to get decent information (although rolls may still introduce randomness).

I believe there is nothing in the official rule books specifically for this kind of thing, but if there is, a quote would be great.

Answer 1

I believe there is nothing in the official rule books specifically for this kind of thing,

I believe Advantage could be something you can use here:

(DMG 239)

Advantage and Disadvantage

... Characters often gain advantage or disadvantage through the use of special abilities, actions, spells, or other features of their classes or background. In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result.

If the character making the check is proficient in both Arcana and Investigation, simply grant Advantage on the check to figure out the spells that the Helmed Horror is/are immune to.

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This works out well because your requirements are:

Combine both skills such that both skills are relevant

Having both skills is relevant, it grants advantage which is roughly a +4 to the check.

More specifically, I want a character unskilled in either skill to have significantly lower odds of getting good results.

Having only Arcana or Investigation takes away Advantage, getting the PC significantly lower odds of getting good results.

Both skills should be required to get decent information.

Try applying a graduated DC here, I use it for some checks in my game all the time, perhaps something like:

DC 10 - You know Helmed Horrors are made immune to some spells, you just don't know which.
DC 15 - Examining the glyphs on its armor, you also figured out one spell its immune to...
DC 20 - You know an additional spell it's immune to...
DC 25 - You know all 3 spells it's immune to...

Depending on what you mean by "both skills are required to get decent information", simply granting Advantage isn't a good fit for this, as with a lucky enough roll (roughly 10%, with a 19-20 result, depending on the PC's stats and level), someone who is proficient with just Arcana or Investigation could get a 25, though much less likely than if they had Advantage (which improves the likelihood of a 19-20 roll to 19%).

Answer 2

I would just split the roll - investigation to inspect, find the runes and magic designs and deduce they are a specific enchantment, and then arcana to know what they do. That would also enable the group to succeed with split proficiencies - one character discovers the enchantment, alerts another, who puzzles out the effect.

Answer 3

In my Group, we use a "Skill Challenge" to make the process of resolving laborious tasks, that have many characters and "angles of attack" in an interesting fashion for everyone at the Gaming Table.

The "extended, collaborative check" in itself is no new concept: With this, we did everything for years. From Building a barn, repairing ancient machinery or investigating a crime from different angles (and much more)

Let me Explain the mechanic and their intention:

• Have an idea how many "successes" the Players need to perfectly resolve the Situation at hand.
• Set the Standard DC for any given Check, as a fallback point.
• Let the Players come up with Ideas to use any appropriate skill (persuasion to ask the local academy, Arcana for studying, whatever).
• Collect "successes" (say the DC is 15. So mark 1 Success for 15-19 for Example).
  • Mark +1 Success for every 5-Point Step someone beats the DC. (20,25...)
  • Maybe allow the Help Action for Advantages(?)
• See the number of Successes and from there decide what they can find out. (In this particular Question: One Spell per Success?!)

More things to consider (that will take you like 2 Minutes when you have done it once):

• How many checks can anyone player make? (Should be equal so everyone has the same chance to completely make or break his part)
• How much time does one a check cost the group? (So maybe you want to limit the attempts by this)
• what story consequence on a single failure, or a complete failure?
  • If someone does not meet the DC, maybe give the next character a Disadvantage, and find out why the previous check influenced this one despite them happening half the city away.
  • Players can describe their ideas to use unusual checks: Make athletic for Investigation by impressing someone at a bar and carouse later with them, or maybe Set the Stage to give another Player an Advantage for his check: Like Intimidate the mark in an alley, and another character shoos the first one off. So his Persuation Check gets a Bonus/Advantage.
  • Gradient results: Assumption: The players get ALL info with 10 successes. So make it less (or faulty) by 8/6/4 and so on.

How this mechanic feels and what Players can contribute

You can paraphrase most of these little scenes and get a longer thing done with a few Sentences per Check, and make the Usual: "One Week later" cut more interesting, and everyone on the Table can contribute with their Ideas, what little no name NPC they want to get, or where they will break in to look at whatever they need.

So you as a GM can give the players a little leeway to improvise their approach and the set they do it in (GM Veto there if needed, so they don't break the Set or Plot).

Example: So they can Break and enter somewhere. They WILL get out of it unscathed, but did they get what they want?

If they were surprised (did not beat the DC) so someone noticed, and the next player that wants to Talk to a Nobleman gets a disadvantage because "the Streets are not save, someone broke into Lord McGuffins House last night" and so there is general suspicion.

This makes for a lively resolution of otherwise dull dice rolling orgys, at least in my experience of the last 8-10 years.

(Examples that use some Variant of it in their core rules: D&D4, Dark Heresy, nWoD, Buffy, The one Ring (social combat) )