Gauntlet, in both the original release of the game ("Pre-Slayer Edition") and the Slayer Edition update, has a total of four difficulties: "Easy", "Normal", "Hard", and "Unfair". Though all four difficulties are present in both versions, the Slayer Edition update changed each of their icons in Campaign. "Easy" now uses the icon that was formerly the "Normal" icon and so on—with the former "Easy" icon now being unused, and "Unfair", as an entire difficulty, now removed from Campaign (but still present in Endless).
Slayer Edition also seems to have introduced a very minor "pros and cons" system in the difficulty descriptions seeing as how +'s and -'s are used as bullet points instead of only -'s, as was the case in the original release.
Enemies take 200% damage. It may be worth noting that "Easy" appears to slow down the gameplay itself in some regards relative to the other difficulties, as can be observed with the use of Lilith's hammer attack. In Endless, "Easy" is the only difficulty not represented.
In Endless, the "Normal" difficulty is represented by floors 1-18.
Enemies take 70% damage, and players take 150%. In Endless, the "Hard" difficulty is represented by floors 19-36.
Prior to Slayer Edition, the highest difficulty in Campaign went by the name of "Unfair". With the addition of Slayer Edition, this difficulty was removed from Campaign (and is no longer mentioned in the game at all by name), however, the difficulty itself can still be experienced in Endless on floors 37+. Although this difficulty affects player and enemy damage in the same way as "Hard", it can perhaps be most easily recognized by the food having been turned to gold.