Cheapest priced copy (Tarkir: Dragonstorm Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Barrensteppe Siege 
Enchantment · Rare · Tarkir: Dragonstorm Promos · #171p
As this enchantment enters, choose Abzan or Mardu.
• Abzan — At the beginning of your end step, put a +1/+1 counter on each creature you control.
• Mardu — At the beginning of your end step, if a creature died under your control this turn, each opponent sacrifices a creature of their choice.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
4 printings · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Tarkir: Dragonstorm Promos | 171p | Rare | $0.49 | $0.49 |
| Tarkir: Dragonstorm Promos | 171s | Rare | — | $0.79 |
| Tarkir: Dragonstorm | 171 | Rare | $0.49 | $0.49 |
| Tarkir: Dragonstorm | 384 | Rare | $0.99 | $0.99 |
2 rulings from Wizards of the Coast
- 4 April 2025If you somehow control Barrensteppe Siege and no choice was made for it (perhaps because another permanent on the battlefield became a copy of it), it has neither of the two abilities.
- 4 April 2025Barrensteppe Siege’s Mardu ability will check as the end step starts to see if a creature died under your control this turn. If none did, the ability won’t trigger at all.