Pit Lord is an amazing pusher with his strong AoE making him useful all stages of the game, his skills are not direct target spell but he is still a great harasser. He is mana hungry early game until you get soul ring, by then he might still be a bit mana hungry.
Soul Ring and Boots of Travel are very useful on Pit Lord allowing him to use his spells more which helps harass and push, boots of travel allow him to get away quicker than dark rift when getting ganked while pushing and its a much shorter cooldown. Pipe of Insight gives him magic resist and health regen while giving a nice buff for the whole team during team fights while Vanguard helps block damage too while giving more health regen. Black King Bar allows you to channel your ultimate without being interrupted by getting stunned or silenced. Finally Assault Cuirass gives you armor and attack speed while giving your allies attack speed and armor as well, which provides survivability and damage output during team fights and pushing.
Firestorm is Pit Lord's main damage spell giving off a large AoE damage and leaving a 3 second burn on any target it hits. pit of mallace is a great escape tool because it snares anyone that walks in it while dealing a little damage which could give you enough time to use Boots of Travel to teleport out of the situation. Expulsion is great for after you killed some minions or monsters and you can use it to kill the rest and maybe even harass people like Drow Ranger sitting in the back farming. Finally Dark Rift is Pit Lord's ultimate allowing you and any nearby allies to teleport to any position on the map after a short delay, very useful in starting or escaping a team fight.