I have a player who is playing a MtG race and I thought it'd be fun to have his character's spark activate and have him become a plainswalker.

This wouldn't replace his class, but be an additonal feature he gains.

I spoke with him and he loves the idea and he prefers to be a white/black mana plains walker.

I don't know a lot about MtG, but from my research I know that Plainswalkers usually have a resource building ability, a spending ability and an ultimate move. Building on the White/Black theme this is what I have so far.

Your mortal body has suffered a debilitating blow - however your bravery and sacrifice has awakened your spark!

Mana builder:
Every time you make a successful hit, roll 1D6,

On a 1 or a 2, you gain 1 White Mana.
On a 5 or a 6, you gain 1 Black Mana.

You may have up to 5 total mana and they can be of either color.

When rolling initiative, you gain one mana of your choice.

Mana Spender:
When you have 3 mana of either color at the start of your turn, you may use a bonus action to cast a spell.

For 3 white Mana, you can cast cure wounds at 3rd level.
For 3 Black Mana, you can cast Inflict wounds at 3rd level.

If you spend all 5 mana (min 3 of the same color, remainder can be either color) you can upcast the spell as a 4th level spell instead.

After combat, the Mana returns to the Aether and your mana is returned to zero.

Special Ability:
Once per long rest, you may cast Sanctuary.

Right now a 3 and 4 on the D6 gives nothing, but rolling nothing kind of sucks, so I considered making it roll a 3 gives a mana of the players choice, and 4 gives 2 mana of the players choice, or 3 gives 2 white and 4 gives 2 black, or leaving it as nothing.

What do you think? I don't want it to be super OP by giving a free heal or damage spell too often - Since he's playing a Monk, he could hit multiple times per turn. So sometimes he'll build mana really fast, other times he may not build mana until near the end of the fight which could be good to have a clutch heal or damage spell in his pocket.

But how balanced would this be?

What class are they playing?

They are playing a Monk, but I wanted to make it kind of open in the sense that any class could become a plainswalker.

Then I wouldn’t tie the resource gathering to hits because spellcasters often don’t get any. Lots of times they cast spells that buff party members, control the terrain, or force saving throws instead of attacking. How about letting them gather 1d2 mana every turn as a bonus action, or if you cut it to an 1ce/day feature even simply at the beginning of their turn? Maybe something like this:

Plainswalker (Bk/W)

Gather Mana:
Whenever you roll initiative you can activate this feature, it stays active for 1 minute. When you do so, you gain your choice of 1 black or white mana. Once you activate this feature you cannot do so again until you finish a long rest. After 1 minute your mana count returns to 0.

At the beginning of each of your turns you gather your choice of 1d2 black or white mana, to a maximum of 5 mana of any combination of colors

Spend Mana:

You can use your bonus action to expend mana and cast a Plainswalker spell. For 3 white mana, you can cast cure wounds as a 3rd-level spell. For 3 black mana, you can cast inflict wounds as a 3rd-level spell. If you spend 2 additional mana of any color, you can instead cast these spells at 4th-level.

Special Ability:
As an action you can spend an action to cast the sanctuary spell. Once you cast this spell in this way, you cannot do so again until you finish a long rest.

You cast your Plainswalker spells using Wisdom as your spellcasting ability, and without requiring Material components.

Actually, now that I’ve thought about it more I would reorganize it like this to make it more “D&D.”

Plainswalker (Bk/W)

As an action you can cast the sanctuary spell as a Plainswalker spell. Once you cast this spell in this way, you cannot do so again until you finish a long rest.

Tapping Into the Weave. Whenever you roll initiative you can activate this feature, it stays active for 1 minute. When you do so, you gain your choice of 1 black or white mana. Once you activate this feature you cannot do so again until you finish a long rest. After 1 minute your mana count returns to 0. At the beginning of each of your turns you gather your choice of 1d2 black or white mana, to a maximum of 5 mana of any combination of colors You can use your bonus action to expend mana and cast a Plainswalker spell of your choice. For 3 white mana, you can cast cure wounds as a 3rd-level spell. For 3 black mana, you can cast inflict wounds as a 3rd-level spell. If you spend 2 additional mana of any color, you can instead cast these spells at 4th-level. For 1 white mana and 1 black mana you can cast the sanctuary spell.

You cast these Plainswalker spells without Material components, and using either Intelligence, Wisdom, or Charisma as your spellcasting ability (chosen when you gain this boon).

Wow. Thanks for the feedback. I really like the final revision.

For 1 white mana and 1 black mana you can cast the sanctuary spell.

Are you thinking that they can spend 1 white and 1 black to cast sanctuary in addition to the once per long rest?

Thanks! I’m glad you like it. Happy to help. Just do me a favor and add that missing period in when you use it. 😂😂 I used to play a lot of M:tG, and this fees pretty right to me. Things like “tapping,” random mana gains, phrases like “mana of any color,” and spells that require multiple mana types (gold cards) are all nods to that game, while the layout and actual mechanics all fit D&D.

Yes, I was. They get 1 freebie per day that they can use whenever, and then while this Tapping Into the Weave feature is active they can spend mana to cast it as well. It’ll cost them a temporary resource (mana) to do it, which means not casting one of the other two spells instead, so it seems fair. And it gives them something specific to do with mixed mana.

I was thinking about it and if you wanted to give it a little more oomph you could add in letting them cast bless for 2 W or bane for 2 Bk as 2nd level spells. That would also fit the theme you’re going for and give the player a few more options. Options make it more powerful, but the overall power level of the boon wouldn’t really increase that much.