The purpose of this deck is simple: flood your deck with 4/4 spider ambushes and use strong draw mechanics to keep playing them for free, gaining a massive tempo advantage. While using these to chip away at the enemy's life total, you can summon the darkness as a final threat that can come into play immediately upon an unfavorable draw.
When possible, after playing Fal'dorei Strider, Shadowstep it and replay it the next turn. This combo will put 8 free 4/4 spiders into your deck, which equates to about a third of the cards in your deck. As they draw a card when drawn, the ambushes can trigger off of each other and chain multiple 4/4s while not interrupting your draw tempo or costing any addition mana. To put the level of value in this combo into perspective, the three cards to set this up cost 12-mana over three turns [4+(2+2)+4] but produce a maximum of 54/55 in future stats with no additional mana investment.
Do not play The Darkness on curve because that will destroy any tempo you are trying to create. Play him AFTER you play Valeera the Hollow and use her power to create a copy and send an additional 3 Darkness Candles into your opponent's deck. This will double the reward of drawing the 3 candles while halving the average time it takes to do so.
It is important to note that this is NOT a mill deck nor do you want the opponent to mill cards once the darkness is on the field. You will outrace most decks to fatigue, albeit while summoning an unreasonable number of spiders and having the backstop of your weapon to prevent damage.