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As explained in How does an arcane focus work with an Eldritch Knight?, Eldritch Knights can't use an arcane focus as a spellcasting focus.

Ruby of the War Mage (XGtE p. 138), however, states:

Etched with eldritch runes, this 1-inch-diameter ruby allows you to use a simple or martial weapon as a spellcasting focus for your spells. [...]

This paragraph doesn't mention anything about arcane foci or anything like that, but straight up says that you can use the weapon as a spellcasting focus; the only requirement being that you are a spellcaster (which is true for Eldritch Knights).

Can you use this magic item to circumvent the otherwise existent restriction to component pouches for Eldritch Knights? Or am I missing some rule that disallows this "trick" / workaround?

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  • \$\begingroup\$ One of the answers from the linked question does point out that using the Ruby-ed weapon does require an attunement slot, so while not disallowing it, it is a cost. \$\endgroup\$
    – Foon
    Commented Jan 19, 2021 at 20:10
  • \$\begingroup\$ @Foon true, that will be a noticeable disadvantage at higher levels (shouldn't be much of an issue below level 5 unless you're playing a magic-item-rich campaign). Having to dedicate a free hand to a component pouch is worse, though, in my opinion. \$\endgroup\$ Commented Jan 19, 2021 at 20:37

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Yes, the Eldritch Knight can use a Ruby-ed Weapon as a focus

You don't need a special feature to use a spellcasting focus. You normally need a feature which lets you use X item type (arcane or druidic focus, musical instrument, tools, sword, etc.) as a spellcasting focus. The ruby comes with its own feature letting you use it.

The general rule which covers the use of spellcasting focuses are in the Component Rules of spellcasting

A character can use a component pouch or a spellcasting focus (found in chapter 5, “Equipment”) in place of the components specified for a spell.

Player's Handbook; Chapter 10: Spellcasting; Casting a Spell; Components; Material (M)

However, by default there isn't anything which is always a spellcasting focus. That gets defined by a feature (or subfeature of the Spellcasting feature) for the specific caster.

The Ruby would be one such feature (creating an additional, specific rule for the relevant character) which lets you use the specific weapon as a spellcasting focus. Perhaps of note, it is not the only feature which allows a weapon to be used as a spellcasting focus, see the College of Swords and their Bonus Proficiencies feature (also from XGtE).

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  • \$\begingroup\$ I found the phrasing in your first paragraph awkward. I think you're arguing that anyone can in theory use a focus, but if your class rules don't provide any kind of item that can be a focus, you in practice can't. And thus all that's needed is a rule to let you use a specific type of item as a focus, even if you previously had zero such options. But especially without a phrase like "in theory", saying "you don't need a special feature to use a spellcasting focus" makes it sound like everyone (including baseline EK) could already be using some kind of focus. \$\endgroup\$ Commented Jan 19, 2021 at 15:42
  • \$\begingroup\$ IDK if there's a way to phrase that so the initial reading of the first sentence doesn't sound wrong until finding out in the next sentence where you're going with that. Or I guess it's not "in theory", it's an "in general" / "if they have one" point that you're making. \$\endgroup\$ Commented Jan 19, 2021 at 15:44
  • \$\begingroup\$ May I suggest: While any spellcaster may use A spellcasting focus, they cannot use ANY spellcasting focus. Normally, a feature lets your character use a specific item type as a spellcasting focus such as arcane or druidic focus, musical instrument, etc... The ruby comes with its own feature letting you use it. \$\endgroup\$ Commented Jan 19, 2021 at 18:36

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