Fundament of this guide is to make Challenge Rift just after you create your lvl 1 seasonal character and then spend bloodshards on lvl 1 necro on buing rings from Kadala. If you will be lucky you get your Circle of Nailuj's Evol which you can use since you hit lvl 16 !!!
4th skill slot is totally open ! you can swap bone armor to any curse (like frailty), golem (flesh rune) or bone spirit (aoe explosion rune)...just take what makes you feel more comfortable.
Swift Harvesting passive is also open...for example you may like Overwhelming Essence more or Commander of the Risen Dead if going to drop Bone Armor for Golem.
You can visit me at twitch.tv/psikustv
It's my first guide...hope you'll enjoy it! good luck and have fun in season 12 !