Shadow Knights are unique from all casting classes in that we have no direct way to cause direct damage to our opponents, nor can we heal. The trade off is of course, our lifetap series of spells. This causes a small amount of damage to our enemies and converts that damage into healing for us, however, the price is an extremely high mana cost. One would be best off not using this as a way to damage your opponent in combat, rather, save it till the end of the fight, perhaps when the monster is running away. Also, after the fight is over, and mana is high while hit points are low, consider draining the nearest large rat for some healing.
This is not to say that Shadow Knights are weak casters. We have the most powerful damaging spells of any tank class. Our dot's are extremely powerful for a relatively cheap mana costs. In addition, most of the dot's can be used in conjunction with other dots at the same time, causing an extreme amount of damage, while we are of course hiting the monster with our weapons. Be sure to use your dot's wisely, if you are in a large group and the monsters you are fighting die quickly, the dot's may be a waste of mana, consider the infamous disease cloud instead.
In addition to combat spells, Shadow Knights also have some interesting extra spells, most notably, our pets. Our pets suck, there is no two ways about it. However, this is not to say they are unusable! Our pets to an obscenely low amount of damage to what ever creature you are fighting, as the pet is about half its level. However, if you are fighting in an area, and have full mana, consider summoning your pet. It does some damage, and its not something that needs to be cast in combat. Therefore, you can summon your pet, get full mana, and be ready for combat, with a minor companion by your side. Do not underestimate vampyric embrace, either. This spell causes a minor amount of damage to your foes and a minor amount of healing to you. Sounds pretty worthless, but the higher your dexterity, the more you drain, and the mana cost is virtually free. Feign Death is a spell all Shadow Knight's should have memorized at ALL TIMES. This is our save-your-arse spell. You never know when things will turn bad, or possibly a mangy paladin gets trained to your area. Casting of this spell causes you to appear dead to all npc's (spell failure, other than fizzles, is about 5-10%). Given a little time, the npc will usually leave you alone, allowing you to go about your business. Beware: some monsters get angry seeing the dark warrior they just slayed up and about, you would be wise to zone after feigning death. You will hear other classes whining about how broken this spell is, ignore them, they do not know the price of being evil. A few more worth mentioning: Gather Shadows: invisibility, nuff said; Deadeye: See invisible/infravision, increased knowledge of your hidden surroundings is very valuable; Endure Buffs: downright terrible, best used when you know you will be fighting a necromancer or a shaman; Shadow Vortex: Ok, I am an AC freak, i love monsters without AC, and giving me AC to boot is a great bonus.
As always, these are simply my opinion based upon my experiences. Granted, it is not all-inclusive, nor is it necessarily accurate to your experience. A human Shadow Knight who thinks like I do could be well served using these thoughts : )
Level 9 - This is a major turning point in the life of a young Shadow Knight. These spells will do much to increase your power in Norrath.
Level 15 - Clinging Darkness is our first dot (damage over time), it also slows your target down, a useful tool.
Level 22 - Vampyric Embrace is our first buff worth using. It steals life from your opponent randomly in combat.
Level 30 - Feign Death and Gather Shadows are two of the most useful tools a dark warrior will recieve, use them wisely.
Level 39 - Heart Flutter is another great Shadow Knight combination dot, it lowers your opponents Strength and Armor Class as well as causing damage.
Level 49 - Our final spell level offers a major upgrade to my favorite dot, Dooming Darkness.