Now before hand please take note this will require "bending the rules" to make this actually work but i feel it balances itself out in the long run.
Came up with an idea for an improvised flaming sword essentially.
Being creature with Fire resistance (Tiefling or Fire based Dragonborn). Take Eldritch Knight with Magic initiate Feat. Convince your DM to let you Take heat metal as your spell. You can now heat your weapon up to deal an extra 2d8 fire damage when ever you make contact with another creature and you'll have proficiency on all your con saves to keep hold of the weapon and keep the spell up.
The spell specifically says you can use your bonus action to cause the damage again and that it affects all creatures in physical contact with the item in question. So it would require your DM to let you use your bonus action and main action to simultaneously heat the sword to add fire damage to your attack each turn requiring you pass the con save to keep hold of the weapon.
Pros: Up to a maximum of an extra 7D8 fire damage with a 9th level spell slot added to your attack. RP wise it could add an intimidation factor with you holding a red hot sword and barely flinching
Cons: have to make 2 con saves, you take half of all the fire damage, you have disadvantage on all attacks and checks.
That's not just "bending the rules"; it's actively ignoring rules, and making up new ones.
Out of everything the main thing I want to point out is that the Heat Metal spell simply does not do what you think it does. It does not deal 2d8 fire damage whenever the affected item contacts something (ie, when you attack with it). The only times it causes damage are when you initially cast the spell, and when you use a bonus action to heat the item again. It is not a continual effect; it's a brief surge of heat, and it only affects creature(s) in contact with the item at that exact moment.
If you want an actual flaming weapon, there's obviously the existing Flame Tongue template. There are other very easy ways to create an improvised version that don't require bending any rules. Get a Oil (flask), coat your weapon, and spark it. Alternatively, use Alchemist's Fire (flask), but that will require some DM interpretation.
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You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
You could however play something like a forge cleric and heat up the metal tip of either your crossbow bolts or of any melee weapon with a wooden shaft.
The rules are a little ambiguous about what it would take to "touch" an enemy with your weapon without trying to actually swing it at them. If one were grappling an opponent and holding a dagger that was subjected to Heat Metal, I could probably see allowing the grappler to touch their enemy with it as part of that bonus action. If they were squared off and not already grappled though... the concept of a "touch attack" doesn't exist in 5E, so i'm not sure there is anything official to fall back on. Maybe an opposed Acrobatics check? And even then not sure if that might not justify being a full Action instead of a Bonus so....
Neat idea in theory, but mechanically very very messy and homebrew-y.
As Sigred states, Heat Metal doesn't work like this at all, and you can't take it as an EK. What actually happens needs to be that someone else casts Heat Metal on your sword and then you take damage from it.
If you want a flaming sword as an EK, then you can cast Green Flame Blade as a cantrip and make an attack with it.
Or, splash some Druid to get Flame Blade, although you may have to find some non-metal plate if you still want to be tanky (hey, at least it'll protect you from Heat Metal).
Now before hand please take note this will require "bending the rules" to make this actually work but i feel it balances itself out in the long run.
Came up with an idea for an improvised flaming sword essentially.
Being creature with Fire resistance (Tiefling or Fire based Dragonborn). Take Eldritch Knight with Magic initiate Feat. Convince your DM to let you Take heat metal as your spell. You can now heat your weapon up to deal an extra 2d8 fire damage when ever you make contact with another creature and you'll have proficiency on all your con saves to keep hold of the weapon and keep the spell up.
The spell specifically says you can use your bonus action to cause the damage again and that it affects all creatures in physical contact with the item in question. So it would require your DM to let you use your bonus action and main action to simultaneously heat the sword to add fire damage to your attack each turn requiring you pass the con save to keep hold of the weapon.
Pros: Up to a maximum of an extra 7D8 fire damage with a 9th level spell slot added to your attack. RP wise it could add an intimidation factor with you holding a red hot sword and barely flinching
Cons: have to make 2 con saves, you take half of all the fire damage, you have disadvantage on all attacks and checks.
I think that there are too many problems with this to make it worthwhile, most of which you already know.
I am generally against trying to turn a situational spell into a go to all purpose spell by bending the rules.
That's not just "bending the rules"; it's actively ignoring rules, and making up new ones.
Out of everything the main thing I want to point out is that the Heat Metal spell simply does not do what you think it does. It does not deal 2d8 fire damage whenever the affected item contacts something (ie, when you attack with it). The only times it causes damage are when you initially cast the spell, and when you use a bonus action to heat the item again. It is not a continual effect; it's a brief surge of heat, and it only affects creature(s) in contact with the item at that exact moment.
If you want an actual flaming weapon, there's obviously the existing Flame Tongue template. There are other very easy ways to create an improvised version that don't require bending any rules. Get a Oil (flask), coat your weapon, and spark it. Alternatively, use Alchemist's Fire (flask), but that will require some DM interpretation.
You don't know what fear is until you've witnessed a drunk bird divebombing you while carrying a screaming Kobold throwing fire anywhere and everywhere.
You could however play something like a forge cleric and heat up the metal tip of either your crossbow bolts or of any melee weapon with a wooden shaft.
The rules are a little ambiguous about what it would take to "touch" an enemy with your weapon without trying to actually swing it at them. If one were grappling an opponent and holding a dagger that was subjected to Heat Metal, I could probably see allowing the grappler to touch their enemy with it as part of that bonus action. If they were squared off and not already grappled though... the concept of a "touch attack" doesn't exist in 5E, so i'm not sure there is anything official to fall back on. Maybe an opposed Acrobatics check? And even then not sure if that might not justify being a full Action instead of a Bonus so....
Neat idea in theory, but mechanically very very messy and homebrew-y.
dndbeyond.com forum tags
I'm going to make this way harder than it needs to be.
You basically use your action to make a melee spell attack to deliver the spell ...
like you do with Shocking Grasp.
As Sigred states, Heat Metal doesn't work like this at all, and you can't take it as an EK. What actually happens needs to be that someone else casts Heat Metal on your sword and then you take damage from it.
If you want a flaming sword as an EK, then you can cast Green Flame Blade as a cantrip and make an attack with it.
Or, splash some Druid to get Flame Blade, although you may have to find some non-metal plate if you still want to be tanky (hey, at least it'll protect you from Heat Metal).
dndbeyond.com forum tags
I'm going to make this way harder than it needs to be.