Cheapest priced copy (Commander Masters): $0.99 at Card Kingdom, near mint, their price list as of .
Narset of the Ancient Way 
Legendary Planeswalker — Narset · Mythic · Ikoria: Lair of Behemoths · #278
+1: You gain 2 life. Add {U}, {R}, or {W}. Spend this mana only to cast a noncreature spell.
−2: Draw a card, then you may discard a card. When you discard a nonland card this way, Narset deals damage equal to that card's mana value to target creature or planeswalker.
−6: You get an emblem with "Whenever you cast a noncreature spell, this emblem deals 2 damage to any target."
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.99 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 930 | Mythic | $0.99 | — |
| Ikoria: Lair of Behemoths | 195 | Mythic | $1.79 | $2.49 |
| Ikoria: Lair of Behemoths | 278 | Mythic | $1.99 | $5.99 |
| Ikoria: Lair of Behemoths Promos | 195p | Mythic | $4.49 | $5.99 |
| Ikoria: Lair of Behemoths Promos | 195s | Mythic | — | $6.99 |
6 rulings from Wizards of the Coast
- 17 April 2020An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
- 17 April 2020If a card you discard has {X} in its mana cost, X is considered to be 0.
- 17 April 2020The emblem created by Narset's last ability is colorless. The damage it deals is from a colorless source.
- 17 April 2020Because it's a loyalty ability, Narset's first ability isn't a mana ability. It can be activated only any time you could cast a sorcery. It uses the stack and can be responded to.
- 17 April 2020You can discard a card even if Narset leaves the battlefield before her middle ability resolves. The reflexive triggered ability will still trigger if you discard a nonland card.
- 17 April 2020Narset's second ability goes on the stack without a target. While that ability is resolving, you draw a card and may choose to discard a card. When you discard a nonland card this way, the reflexive triggered ability triggers and you pick a target to be dealt damage. This is different from effects that say "If you do . . ." in that you choose a target after looking at the card you draw and players can respond to the reflexive triggered ability knowing how much damage will be dealt.