Tampered Remnant and the Remnant Form-Change Mechanic
The 
Tampered Remnant in Simply Swords is one of the forms a carried Remnant can settle into — its own note reads "It appears to have been tampered with somehow." It is simply a stage in the Remnant transformation puzzle that ultimately yields a gem for unique weapons.
What a Tampered Remnant Is
Remnants in Simply Swords are unstable magical energy in item form. A 
Contained Remnant is described as "stabilised and contained energy sourced from remnants," and a Tampered Remnant is a variant that same energy can shift into. Both are held items rather than weapons, and both take part in the one slow, self-changing process.
How Remnants Change Form
The mechanic is passive and inventory-based:
- A Contained Remnant or Tampered Remnant carried in your inventory is periodically checked to change form. The rate is the "Remnant transform interval" — higher values check less often.
- Whether it changes at all is tied to the mod's contained-remnant option; disabling unique-weapon drops also stops the change.
- The remnant reacts to its surroundings. Its notes read "It seems to react when near some surfaces," and events fire in play such as "A Remnant in your inventory seems to have changed form," "is acting strangely," and "The Remnant seems to be reacting to a nearby block."
Steering the Transformation
The item's own hints tell you how to influence the outcome:
- "Keeping this item in your inventory may yield further insights" — simply carrying it moves the process along.
- "Try keeping this in your inventory alongside a
Runic Tablet while exploring the End dimension" — location and a Runic Tablet both matter. - Because it responds to nearby blocks and places, travelling through different environments with the remnant on you is how you push it between forms.
Where It Leads
The payoff is a socketable gem, not a penalty. The Remnant's tooltip says to "smith with a Runic Tablet or 
Netherite Ingot to create a gem that can be socketed into unique weapons." So coaxing a Tampered or Contained Remnant through its forms is the road to a 
Runefused Gem you can smith into your Simply Swords unique weapons.