Cheapest priced copy (Strixhaven: School of Mages Promos): $0.99 at Card Kingdom, near mint, their price list as of .
Tanazir Quandrix 
Legendary Creature — Elder Dragon · 4/4 · Mythic · Strixhaven: School of Mages · #240
Flying, trample
When Tanazir Quandrix enters, double the number of +1/+1 counters on target creature you control.
Whenever Tanazir Quandrix attacks, you may have the base power and toughness of other creatures you control become equal to Tanazir Quandrix's power and toughness until end of turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.99 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Strixhaven: School of Mages Promos | 240p | Mythic | $0.99 | $4.49 |
| Strixhaven: School of Mages Promos | 240s | Mythic | — | $2.99 |
| Secrets of Strixhaven Commander | 332 | Mythic | $0.99 | — |
| Strixhaven: School of Mages | 240 | Mythic | $0.99 | $1.49 |
| Strixhaven: School of Mages | 284 | Mythic | $2.49 | $5.99 |
4 rulings from Wizards of the Coast
- 16 April 2021As the last ability resolves, the base power and toughness of other creatures you control are set to Tanazir Quandrix's actual power and toughness, not just its base power and toughness. If Tanazir Quandrix's power or toughness changes later in the turn, the other creatures you control aren't affected.
- 16 April 2021To double the number of +1/+1 counters on a permanent, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.
- 16 April 2021Tanazir Quandrix's last ability overwrites all previous effects that set a creature's power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after the ability resolves will overwrite this effect.
- 16 April 2021Any effects that modify a creature's power and/or toughness without setting them to a specific value (i.e. ones that don't affect base power and/or toughness) will apply after its base power and toughness are set, regardless of the order those effects were created. The same is true for counters that modify its power and toughness.