Dark Knights are technically mages but this one has an axe and therefore counts as a Warrior. Dark Knights are a hybrid class, pretty much a mix of Sorcerers and Knights, who can use both magic and physical weapons. While their strength isn't amazing, it's enough to mess somebody up in conjunction with a few well-placed spells. Some Dark Knights may train as other classes to raise either their strength or magic to give them an edge.
Depending on the person, a Dark Knight may take on an offensive or defensive role. Some favour spells over beating things to death and vice-versa. There is no 'guide' to being a Dark Knight. Their physical defence stat is higher than most mage classes, so they can engage in close range combat where other mages cannot. Rarely, a Dark Knight may focus on improving their defence and act as a stereotypical Knight. You can tell who's a Dark Knight in battle because they wear dark armour and look edgy.
The magic Dark Knights have access to (offensive, conjuration-esque) isn't particularly useful in day-to-day life, but enough to help out with small domestic tasks here and there. For example, they should be able to light candles without too much trouble, but unless they want to shroud the neighbourhood in shadows it's probably best that they stick to a simple way of life. Although they are usually weaker than Warrior-Warriors, pickle jars should not pose a threat to them.