Where is Malevolence in the Warlock talent tree?

Affliction tree, row 4, column 1. You need 15 points spent in Affliction before you can learn it.

Is Malevolence new in WoW Forever?

Yes. Malevolence has no Classic 1.12 counterpart.

Rank	Effect	Source
1	Increases the critical effect chance of your Shadow spells by 1%.	client
2	Increases the critical effect chance of your Shadow spells by 2%.	client
3	Increases the critical effect chance of your Shadow spells by 3%.	client
4	Increases the critical effect chance of your Shadow spells by 4%.	client
5	Increases the critical effect chance of your Shadow spells by 5%.	client

Tree: Affliction, row 4, column 1. Needs 15 points in Affliction first. The earliest you can finish all 5 ranks is level 29 if you put every point into this tree.
Competes with: Nightfall, Curse of Exhaustion in the same row.

Malevolence is new in WoW Forever. Nothing in Classic 1.12 does this for Warlocks.