Shadow-knights are the kings of aggro management, they get plenty of Hate generation AA, be it AoE or single target.
They're as good as Warrior on most Tanking purpose but will lack the Warrior disciplines for the strongest bosses.
It's the easiest tank to use, due to the amount of aggro they could generate.
Solo: Tank classes are usually not meant to play solo, but Shadow-knights can do well, they have a pet (strong as a DoT if you gear them), DoT, Snare and access to strong 2 handers, so can effectively do Fear Kitting. Their Lifetap allow them also to heal themselves and the plate they wear makes them really strong at taking hits, so you can even solo tank.
Group: Shadow-knights work well in any group, you allow a lot of room for mistakes with your almost limitless aggro abilities, so melee or caster group will benefit from your presence, especially strong DPS who lack aggro reduction capacity.
SK are also one of the best pulling class, they get Necro snare line (Darkness), Feign Death, a self Invulnerability (Harmshield) and can split very effectively packs of mobs using snare + FD.
Be careful using Harmshield, as it tend to break bot ini when they try casting a beneficial spell on you. Be ready to reset their macro.
AoE Group SK fit really well in an AE group, Explosion of Spite and Explosion of Hatred allow you to take aggro really fast and keep it for a long time, allowing your Enchanter to start easily their stunlocking as you manage the melee rush.
You can easily pull large pack of mobs with a shield and some defensive AA, use a HoT before pulling (as it doesn't generate aggro to your healer if done before pulling) and Selo's Sonata (BRD AA) to make a safe and fast pull.
Explosion of Hatred, Explosion of Spite → Your 2 AoE Hate generator AA (Explosion of hatred need a target), when you get those, your job will be way easier as you'll be able to keep aggro on all mobs around you. You can add the single target AA like Hate's attraction, but these 2 are the only really mandatory for your job.
Combat Agility, Combat Stability, Shield Block → Those are your main defensive AA, get CA/CS at least to 8 (higher ranks gets costly) before buying other AA.
Planar Power, General Sturdiness, Aromor of Wisdom → more costly, but increase also your tanking capacity.
Touch of the Cursed, Soul Abrasion, Destructive Fury → All those increase your lifetap dmg/healing, getting you more survivability.
Aim for Muramite Armor, as more guild are able to do GoD content, the price is decreasing and muramite is now affordable. It will give you really strong AC/Stats and some good focus, your tank should always be the first to be geared and your main healer second (generally your Cleric as they wear Plate).
Generally speaking, try to get the highest AC item you can get, because your job will be taking hits and it's always better to reduce them instead of having a few extra hp (10 dmg reduction means 100hp gained in 10 hits).
Shielding stats should also be in high priority.