Magic Item: Hag-Soap

How the PCs gain the Hag-Soap varies.  Sometimes they loot it from huts and lairs, and other times they trade for it.  On rare occasions it is offered as a gift.

In all cases, it is a bar of soap crafted by a Hag of one sort or another, and only a desperate fool would bathe with it.  As it happens, the phrase desperate fool is often synonymous with the word PC.

1. Disguise - washing with this soap causes the bather's skin to slough away, revealing skin of a far different color (and perhaps patterned or plaid, as well).
2. Curing - the bather has all of their current diseases and curses washed away.
3. Youth - the years are literally washed away from the bather, to the tune of 3d4 per wash - if the bather loses too many years, they dissolve into egg and sperm.
4. Revival - washing a dead thing with this soap restores it to life, Pet Sematary style.  
5. Cleansing - the soap washes away all impurities - scars, birthmarks, tattoos, moles, skin tags, anything asymmetric about the person.  These impurities later rise as a twisted clone of the bather, hellbent on destroying the bather's reputation and then the bather themselves.
6. Magic - the lather of this soap is full of latent possibilities.  When the last of it is rinsed away, the bather can now work magic - just not well or any degree of control.  In fact, all spells cast by or in the presence of the caster force a roll on a wild magic surge table.  If 5e is your jam, its a free level in wild magic sorcerer.
7. Power - this soap transforms the bather into a powerful werewolf.  Of course, when the change comes over the bather, they become a temporary NPC and their player rolls on this table to see what they get up to.  
8. Charming - the bather using this soap is utterly vulnerable to charm effects, getting no saving throw at all for the next 24 hours.  This soap also has the best smell, so is popular among the more insidious crowd.

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The real question is what becomes of that which is washed away: the skin, the curses, the disease, the death, the normalcy.  If bathing in a tub, and the tub water is drained or thrown out, wherever it lands tends to kill the plant and insect life immediately (a hint) and that area may become a small wild magic zone.

If bathing in a stream or under a small waterfall, the runoff heads downstream, wreaking havoc as it goes.  The worst is when it is a curse that is washed away, as all sorts of creatures end up with it, although diseases that are washed away are almost as bad.

Note that if any of these soaps are bargained for from a Hag, the Hag is likely to recommend bathing in a nearby stream 'as part of the magic' to ensure later chaos.  

Local druids, fey, and other sylvan creatures are likely to become irate with whomever left their waste behind like this.  

A secondary question is what materials go into these soaps.  None of it is wholesome, but it explains what happens to the people the Hags kidnap.