How do I earn Reaper Energy?

As you said, there are weapons which give you reaper energy on attack, on kill, on critical hit or on execute. Those are not (only) end game items. I've had one for level 10 or somewhat which gave me energy on kill. In addition you can equip the talisman from the shaman in the first land (you can get it on a side quest where you have to collect 3 materials. The weapons are just Lucky drops, so good farming.

I found additional Information in the IGN Game Guide. It seems that the killing of enemies with scythes gets you also reaper energy.

While Testing I discovered that you gain reaper energy on every hit on an enemy with your scythes. In addition, it seems that you gain more on kills (it is difficult to say, cause I don't have any log file...)

To answer your entire question:

• You gain reaper energy on every Hit with your scythes.
• You gain more reaper energy on a kill with your scythes.
• You can increase the amount of reaper energy received with stats on your weapon or the Grim Talisman
• You won't gain any reaper energy with other weapons than scythes.

From what I have found after hitting a training posts for about 5 minutes or so, with my secondary weapon (a level 9 axe with 227-253 damage + 48 lightning damage) on an empty reaper energy bar (the spiral to the left of your health, wrath, and xp bars). The secondary weapon wielded no reaper energy. Then with my scythes (Possessed double scythes with no reaper energy bonus) I could already see it going up after a few hits.

I then equipped the Grim Talisman, and tried again. The reaper energy bar filled slightly with each hit of my axe, and even more so with my scythes.

So as far as I can tell, the Grim Talisman's only purpose is to help boost reaper energy.

And of course once you have an image of a grim reaper instead of the spiral you can hit Lb + Rt at any time to become Death's true form.

You get Reaper Energy by dealing damage with Death's scythes