Question: The Curse:

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> **Clockwork Curse**  
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>  All Attack Rolls the cursed target makes replace their d20 roll with a 10 instead of rolling, as per the effect of Clockwork Amulet.
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Extra Details:

* This curse is expected to apply for at least a full in-game day.
* Encounters for the day will come from the Forest or Coastal Encounters (5-10) tables in Xanathar's Guide to Everything.
* The character will be part of a 5-player group.
* The character's attacks are their ideal method of dealing damage.

The Goal:

* This curse is meant to be a setback or gameplay-shift to the player, not a crippling of the player.
	+ I want the player to spend time affected by the curse to work with it or work around it.
* Since the players can decide not to remove it, the beneficial aspect of it should not be too strong.

The Question:

Is this curse too detrimental that it imposes more of a penalty than I expect? Is this curse too beneficial in the long-run? Or is it reasonably fair?

Answer: 
It seems fine. Finding out how it works seems somewhat fun. The player barely hits a sahuagin without needing to roll, then barely hits the next. Seems good, but there has to be a catch. Then they barely miss the armoured hobgoblin, still with no roll, and keep barely missing. There's the catch. At some point they'll get the idea or you'll just tell them about the "always 10" part.

Now you're onto the meat. The players may even discuss the problem. For each new monster type the player might take a test whack. Goblins, bugbears and a bugbear captain? Goblins are a sure hit -- but should they try the bugbears? If you allow heavy tabletalk, what are the odds someone has memorized the exact AC of each monster?

Options are using that +2 dagger (suppose their main weapon is only +1), or maybe a shove action or a grapple. Or a Bless spell (it seems to me the d4 would still be rolled, and added to the 10). They'll probably gain Advantage once before realizing it won't help. Few feats allow for an optional attack bonus, but they might have one. They can Assist another attack. Or try to draw attacks and Dodge (the monsters don't know the player can't hit them). Doing the last two for 1 fight isn't so bad.

You (the GM) can cheat. If 2 rolled encounters in a row only have monsters that guy can't hit, replace one hobgoblin with a softer gnoll.
Seems like a serious debuff
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In terms of what the character can actually do -- you roll a 15 or over like 25% of the time. That makes for a hard fight, but you can burn resources and win it. This curse makes easy fights trivial, and hard fights impossible. You might be able to, as the DM, design around this -- make things harder by giving more low-level enemies, and totally avoiding high level enemies. But that sounds really tedious. 

Also, how will you deal with advantage? I suspect the expected value of a roll with advantage is about 15. If you give them a 15 on an advantaged attack, it might be more manageable (also more fun).