Sustain in Lane Monster Rounds are going to be for farming to get you to late game quicker. Buy Reach only if you are close to having your ult.
Ganking/Mid Game Anytime after you unlock ult start looking for ganks in you adjacent lanes.
Carry Fights This is where the build comes alive. Drop in when they are focused on your team usually wait from above while they spend dashes. If your ult is off CD buy Reach 1st then Duration. If it is on CD buy CD. Buy Torment Pulse if you don't have enough for a Late Game Item and you're desperate.