When building Rek'sai remember to focus on her stealthy and effective ganks.Also,try to build up her ability to deal tons of damage really fast.For example,when you gank mid,you first attack will burrowed with her Q(pray seeker),then you get into lane with your burrowed E(tunnel).The tunnel will give you the ability to precisely attack on her.Then,you will unborrow with her W to knock them up.Finally,attack her with your Q and E(unborrowed).So,to achive that,you should build attack damage and attack speed.Ravenous Hydra is a must-have item.As it concerns attack speed,you don't need to buy a lot of attack speed items cause she passively gets up to 60% attack speed.Lat but not least,rek'sai is really squishy,so you have to build some defense too