Gaesatae are an elite, infantry-killing shock unit - one of the most fearsome in the ancient world.
The Gaesatae (Guy-sah-tay; "Spearmen") are easily one of the most fearsome groups of warriors in the known world. They fight stripped bare but for neck torcs and sometimes their shoes or boots. They fight with longswords, but are named for their incredible skill with javelins; they can hurl them unnaturally long distances and deal great damage with them. Their charge is most ferocious though, causing many lines to simply fold and flee. This is not all solely due to skill and strength, though they are experienced and huge, muscular men. They imbibe a chemical before battle making them seemingly impervious to pain. They will fight to the very end, and are very destructive when employed properly.
Historically, the Gaesatae were Gallic naked religious fanatics. Their skill was a source of terror, and their frightful appearance shook many. Most horrifying, even beyond their size and skill and terrifying cries, was their seeming invincibility. A chemical they used allowed them to brush off and ignore any pain. They were remarked to do things such as rip javelins that had struck them free from their bodies, and hurl them back, and continue to fight with gashes and wounds that would have surely fell a normal man. They were Hannibal's favored Gallic mercenaries, and were used as his personal guards, as they were so devoted and trustworthy, as well as brave and skilled.
These guys are machines. Even without the bonus hit point and fear factor, they would be great troops, albeit over-priced, and those two bonuses make them monsters to be reckoned with. Do not regard them as invincible - though they will crack most units one-on-one - they can still be taken out the same way most barbarian units are, with a fair bit more effort: missiles and (spear-wielding) cavalry charges to the flanks and rear. However, they remain a great addition to any force and are comparable to cavalry charges for starting a rout.