Sneaky and manipulative, Shadowfists corrupt the teachings of Gorn to justify their sinister world view; The massacre of all those who stand against them through any means neccesary. They combine unarmed fighting with the dishonorable tactics usually favored by Rogues and Assassins. Like all Prestige Classes, they retain the original class's Prime Requisite, in this case, Dexterity. Shadowfists have strayed too far from the light to ever become Priests, like Monks can, but they can become Assassins, albeit later than Rogues. Shadowfists masters can sometimes be found training in inhospitable environs.
Shadowfists in General
Shadowfists trade some good skills for their evil ones. Gain: a lifesteal and critical strike attack as well as rogue flee skills. Lose: all healing, status abnormality cure, and awen spells, also notably they lose arrow dodge. They don't get bear stance or transfer life. Also to take advantage of their hero101 skills they must train demonic intent which keeps them evil aligned.
Be evil aligned, be a monk and kill a monk in Sir Michael's Stronghold. You must have 18 base int at lowmort, or 20 int at hero. Any race can become a shf with the use of QP. Some popular choices are gargoyle and golem. Orc, ogr, or giant would also be good choices (but will need quest points to bypass the int requirement) - basically anything that would be good for a Monk (drg, grf, trl, kzn, etc). Also, note that the amount of hp gained from vamp/vital attacks has been limited to less than it once was, thus reducing the benefit of a brute race somewhat.
The monk you need to kill in order for Diryn (Shf trainer) to accept you is the "meditating monk." The directions to this monk from Marcus is e2s2e2n.
Shadowfists at Hero Tier
Train Wis first to get more practices per level, followed by Dex(Prime Requisite), Con, Str and Int(You can choose to train Int earlier as it will make practicing cost less).
Trainer is Diryn. Concentrate on inclass spells and abilities. All of 'slist' and what you see in 'hlist inclass'/'llist inclass'
Take advantage of your different stances as much as possible. I like to use emu stance while my hp are full, and start combat with a surprise attack, then kick like crazy. After losing 500-1000 hp, I will usually switch to vampire fang stance and start using vs in combat. Spectral stance is useful in areas where the mobs seem to parry more than usual.
A note regarding counters: if you choose a counter other than basic, such as counterkick, you only use that counter against the mob you are directly fighting. If you are fighting more than one mob you will use your "basic" counter on the ones you are not directly fighting. For monks and shadowfists who have not learned vital counter this is a simple counter hit. For shadowfists who have learned vital counter, vital will become their "basic" counter.
Something useful to consider regarding the use of vampire fang is that the damage you deal is much greater for vital strikes used to open combat than those used between rounds. Also, in cases where you kill your opponent quickly it's generally better to open with a vital shot while using vampire fang so that you actually benefit from the stance.
Shadowfists at Lord Tier
Shadowfists serve as tanks at lord tier. They get a bit more hp than monks but tend to get hit significantly more. The reasons include lack of blind devotion and their evil alignment. Also many lord mobs are elementals and thus vampire fang doesn't work well with them. They do have one "trick" which they obtain at lord 325, which is countercrush with vital counter and full counter, which allows them to do non-negligible amounts of damage to mobiles which are susceptible to vital counter, such as gith in Outland.
Shadowfist Remort Options
Shadowfists can remort to assassins at lord 150 (300 to keep race).