Infernal Bolts are projectiles fired out of a crossbow, usually by paladins. They look like normal bolts but are mostly red. They are the most powerful ammunition in the game. Assassin Stars are better but they count as Distance Weapons, not as Ammunition.
An article from Tibianews, published 21. october 2007, suggests that the attack rate for Infernal Bolts is 43. Power Bolts with 40 attack (cost only 7 gp per shot) and Assassin Stars with 65 attack (cost 33 gp per shot) render infernal bolts an unnecessary expensive ammunition. If you can't buy Infernal Bolts for less than 20 gp you should prefer hunting with Assassin Stars, you can wear a shield and deal about 15% more average damage than Infernal Bolts. The only situation where you should pay more is when you really need the higher range (6 sqm with Royal Crossbow instead of 4).
<spoiler>You can trade Soul Orbs for infernal bolts through the The Sweaty Cyclops Quest.
You can obtain 100 Infernal Bolts in The Ultimate Challenges Quest Warlord Level.</spoiler>