What it does
Curse does one of two completely different things:
Ghost-type version: The user loses half their max HP to inflict the Curse status on the target. The Cursed target loses 1/4 of its max HP at the end of each turn. The only way to clear it is to switch the target out.
Non-Ghost-type version: The user's Speed drops by one stage, and their Attack and Defense each rise by one stage. For a slow physical tank, this is a clean trade — both offensive and defensive stats improve at the cost of Speed the Pokémon probably didn't need.
The typing on the user determines which effect fires. Trevenant is Ghost/Grass — it uses the Ghost version. Mudsdale is pure Ground — it gets the stat-change version.
Who runs it
Trevenant (12% usage) uses the Ghost version. High bulk means it can survive the 50% HP cut and still function, though the cost is real. This is a 'trade my longevity for constant drain on theirs' tactic — most valuable against high-HP bulky targets that are hard to KO with damage alone. The Curse Trevenant places deals four turns of drain before knocking the target out mathematically (barring recovery).
Mudsdale (7% usage) uses the non-Ghost version. Base 35 Speed means the Speed −1 is nearly meaningless in practice — it's already moving last. The Attack +1 and Defense +1 stack cleanly on top of its already-high Defense, and with Stamina (Defense rises when hit by physical moves) the bulk scaling after a few turns becomes extreme. Under Trick Room, Mudsdale's low Speed is an asset; Curse's Speed drop doesn't matter at all in that context.
How to use it
Ghost version (Trevenant): Use Curse when the field state is stable and you can afford the 50% HP cut. Targeting the opponent's core — especially a high-HP setup sweeper that intends to stay on the field — maximizes the drain value. The opponent can switch to clear it, but switching costs position and tempo, which is the real friction Curse creates.
Non-Ghost version (Mudsdale): Use it in a safe turn when your partner draws attention with Follow Me or Rage Powder. Accumulate the Attack and Defense boost, then output. One use is already meaningfully strong; two stacks push the numbers to extremes.
Tips & strategy
The Ghost-version Curse is cleared only by switching the Cursed Pokémon out — it's a unique status condition that cannot be cured by Heal Bell or Aromatherapy (those heal standard status conditions; Curse is a different mechanics layer).
For non-Ghost Curse on Mudsdale under Trick Room: Speed −1 is irrelevant in reversed turn order, Attack and Defense boosts are permanent until it switches. The only down side in that context doesn't apply at all — Curse is a pure stat-gain move in Trick Room.
In doubles, the Ghost version's 50% HP cost is significant. Trevenant at 50% HP remaining has reduced survival margin. Plan around what it needs to do after Curse lands — if it just needs to absorb another hit and pass tempo to its partner, that may be fine.