Cheapest priced copy (Hour of Devastation): $3.99 at Card Kingdom, near mint, their price list as of .
Obelisk Spider 
Creature — Spider · 1/4 · Uncommon · Hour of Devastation · #141
Reach
Whenever this creature deals combat damage to a creature, put a -1/-1 counter on that creature.
Whenever you put one or more -1/-1 counters on a creature, each opponent loses 1 life and you gain 1 life.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $3.99 to $13.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Hour of Devastation | 141 | Uncommon | $3.99 | $13.99 |
| The List | HOU-141 | Uncommon | $4.49 | — |
7 rulings from Wizards of the Coast
- 18 April 2017If a creature with wither or infect deals damage to a creature, the controller of the creature with wither or infect puts that many -1/-1 counters on the second creature.
- 14 July 2017If Obelisk Spider deals combat damage to a creature at the same time Obelisk Spider is dealt lethal damage, its first triggered ability will put a -1/-1 counter on the other creature. However, because Obelisk Spider has left the battlefield before that ability resolves, its last ability won’t trigger.
- 14 July 2017If you put one or more -1/-1 counter on each of multiple creatures at the same time, Obelisk Spider’s last ability triggers once for each of those creatures.
- 14 July 2017In a Two-Headed Giant game, Obelisk Spider’s last ability causes the opposing team to lose 2 life and you gain 1 life.
- 14 July 2017If Obelisk Spider deals combat damage to a creature but that creature isn’t on the battlefield as Obelisk Spider’s first triggered ability resolves, most likely because Obelisk Spider killed that creature already, that ability won’t put a -1/-1 counter on it.
- 14 July 2017Because damage remains marked on a creature until it’s removed as the turn ends, the counter Obelisk Spider’s first triggered ability puts on a creature may cause previously marked damage to become lethal.
- 14 July 2017Obelisk Spider’s first triggered ability puts only one -1/-1 counter on the creature, no matter how much combat damage it dealt.