Salt-Upon-Wound’s Power Fist can be acquired at the end of Fallout: New Vegas’s Honest Hearts DLC, either by killing Salt-Upon-Wounds or by getting it in the rewards chest.

Quick tips before you go get the weapon:

• Honest Hearts DLC required. To get this weapon, you must have the Honest Hearts DLC installed.
• Best to take it from the rewards chest. The weapon may be in poor condition, or you may not be able to get it at all depending on which ending you get, however, no matter what, you can always get it from the rewards chest at the end of the DLC. 
• Cannot be acquired in the Chaos In Zion ending. If you kill a character integral to the story of the DLC, the Chaos In Zion will trigger. It’s not possible to acquire this weapon during this ending.

Acquisition

First things first: There is a chance that, depending on your chosen ending, you might fight Salt-Upon-Wounds or otherwise have the chance to kill him. You may do so if you choose, but make sure you don’t pick up the weapon from his corpse. That way, you’ll get a good condition one inside the rewards chest.

The rewards chest will be right by the location “Southern Passage” once the ending slideshow for the DLC ends. It should be directly in front of you when that happens.

Simply take the weapon from the chest, it should be near maximum condition.

Comparing Salt-Upon-Wound’s Power Fist to the Power Fist

• Salt-Upon-Wound’s Power Fist deals 5 more damage per strike (45 damage vs 40 damage).
• It attacks faster (1.36 attacks/second vs 1.09 attacks/second).
• It takes fewer AP to attack with in VATS (25 AP/attack vs 28 AP/attack).
• It has a higher durability (495 attacks before breaking vs 395 attacks before breaking).
• It has a permanent poison effect, which deals 3 damage/second for 10 seconds to any enemy you hit with the weapon. This damage does not stack.

Gameplay Tips

• The poison damage over time effect works best against enemies you can’t hit consistently, or that have enough HP for you to fight them for at least the 10 seconds that the poison is effective for. 
• There is no perk or skill that boosts the poison damage of this fist, meaning that you’re most likely better off with another weapon, like the Saturnite Fist Super-Heated from the Old World Blues DLC, and using that weapon in conjunction with a good stock of poison to be used against bosses.