Cheapest priced copy (Mystery Booster 2): $5.49 at Card Kingdom, near mint, their price list as of .
Comet, Stellar Pup 
Legendary Planeswalker — Comet · Mythic · Unfinity · #166
0: Roll a six-sided die.
1 or 2 — [+2], then create two 1/1 green Squirrel creature tokens. They gain haste until end of turn.
3 — [−1], then return a card with mana value 2 or less from your graveyard to your hand.
4 or 5 — Comet deals damage equal to the number of loyalty counters on him to a creature or player, then [−2].
6 — [+1], and you may activate Comet's loyalty ability two more times this turn.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
5 printings · $5.49 to $10.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mystery Booster 2 | 79 | Mythic | $5.49 | — |
| Unfinity | 166 | Mythic | $8.99 | $6.99 |
| Unfinity | 275 | Mythic | $10.99 | $6.99 |
| Unfinity | 452 | Mythic | — | $9.99 |
| Unfinity | 526 | Mythic | — | $9.99 |
11 rulings from Wizards of the Coast
- 7 October 2022Each + loyalty symbol in Comet's abilities means to put the indicated number of loyalty counters on him. Each – loyalty symbol in Comet's abilities means to remove the indicated number of loyalty counters from him. The only difference is Comet uses these symbols in the effects of his abilities rather than the costs. Good boy, Comet.
- 7 October 2022Something in the game must tell you to roll a die. If you roll a die for any other reason (to simulate a coin flip, to choose pizza toppings, to create alternate timelines), that roll doesn't count.
- 7 October 2022In some unusual cases, Comet may gain loyalty abilities other than the one he normally has. In those cases, the effect of rolling a 6 applies only to his normal ability.
- 7 October 2022Each die is identified by the number of faces it has. A six-sided die is a die with six equally likely outcomes: 1, 2, 3, 4, 5, and 6. The roll must be fair. Although physical dice are recommended, digital substitutes are allowed except in cases where the physical die is required for the effect.
- 7 October 2022If you roll a 3 but there aren't any cards with mana value 2 or less in your graveyard, you'll just remove one loyalty counter from Comet. Sorry, boy.
- 7 October 2022If a die is rerolled, the original roll essentially never happened: it doesn't cause any abilities to trigger, and no effect that cares about die rolls will consider it.
- 7 October 2022None of Comet's abilities require a target. All choices for cards, creatures, or players are made as the abilities resolve.
- 7 October 2022If an ability triggers "whenever you roll a die," it will trigger whenever you roll any die, including the planar die. This is a change from previous Un- rules. Some abilities use the result to determine part of the effect. If you get a non-numerical result (currently just the planar die, but the future is long), that part of the effect won't do anything.
- 7 October 2022Results can be numbers not ordinarily possible on a six-sided die. Spells like Scooch can change the result to 0 or 7, for example.
- 7 October 2022Some effects may modify the result of a die roll. This may be part of the instruction to roll a die, or it may come from other cards. Anything that references the "result" of a die roll is looking for the result after these modifications.
- 7 October 2022If you roll a 4 or 5 while Comet has one loyalty counter on him, Comet will deal 1 damage to a creature or player and then that loyalty counter will be removed. Comet will then go live on a farm, where he'll be very happy but will miss you terribly.