Shadowknight pets can be used to send into battle before you enter the fight. Around this time your pet should will die and the mob will rush you. When it does you meelee it to death, gaining assistance from the DoT spells. At level 15 you get the mend bones spell which allows you to heal your pet, however your better off just letting it die. They are a nice addition to the spellbook but you will not be using them very often.
Same thing goes for endure cold.
Invisibility spells are very useful especially if you are soloing. At level 9 you will be able to cast invisibility to undead which will be useful in primarily undead zones such as Befallen or Unrest . At level 30 you will be able to make yourself invisible to the rest of the world. This will allow you to be able to travel off the mainland as you will be able to sneak in past the guards. It will also allow you to be able to sneak by the monsters you do not wish to fight and get right to the juicy stuff.