Cheapest priced copy (New Phyrexia): $0.35 at Card Kingdom, near mint, their price list as of .
Dementia Bat 
Creature — Phyrexian Bat · 2/2 · Common · New Phyrexia · #55
Flying
{4}{B}, Sacrifice this creature: Target player discards two cards.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| New Phyrexia | 55 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 1 June 2011You can’t force an opponent to discard a card they cast by activating Dementia Bat’s ability in response. The spell is on the stack by then.
- 1 June 2011Unlike most abilities that force a player to discard cards, this ability may be activated whenever you could cast an instant, including during your opponent’s draw step after they have drawn a card.