Take adv of the mana passive
You can trade out Ancient for Totem of Death for extra pen (even Ancient Signet). Blinking Abyss is good for survival but if you don't need it, change it out for either Totem or World Stone for extra mana/ult cd. If you really want to be risky and you're surviving, trade out Ancient Signet for Necronomicon (or Soul Reaver, but build it second for early stacks). This is a low CD build, so be mindful of ability usage.
If you're able to survive and feel ahead. Risky but gives a lot of power if you can have 3 or more stacks.
If you need the extra pen.
Probably won't want to change this if you have Necronomicon, but if you don't feel you need the "get out of jail" card can trade out for extra mana/ult cd
