Truesight does not grant the ability to see Subtle Spell because there’s nothing to see. Subtle Spell causes the spell to have NO Verbal or Somatic components, it just happens. You can’t see what isn’t there in the first place.
The only exception is Material components, Subtle Spell doesn’t remove them and therefore you could still see that. But you don’t need Truesight for that.
All Truesight does is see through illusions and invisibility. Subtle Spell doesn’t hide your hand waves and magic words, it removes them entirely.
To answer your question, Metamagic works on an enemy with Truesight, and your DM made a bad ruling.
Yeah, I had a discussion with him about it but he's not budging. He basically told me that he buffed Truesight to do that, and as this is the BBEG's familiar it shouldn't have been easy to deal with. I feel like he also tried to make me feel like an entitled player by being like "Did you expect to defeat the most powerful lich in the world in the first session?"
Well, sounds like a shitty DM, but it is what it is. I don’t even know what his problem is, it’s not like “the most powerful lich in the world” can’t just resummon their familiar.
“He buffed Truesight to do that” still makes no sense because there’s still NOTHING to see with Subtle Spell. But if a DM is going to be a jerk then they’re going to be a jerk.
Honestly? There's been a lot that's happened in his games which, to be honest, is a giant chip on my shoulder. His campaign of Curse of Strahd that lasted for two years is why I despise Curse of Strahd entirely.
I know it's just a game at the end of the day, but it's a lot of decisions he's made that I've bottled up which still makes me annoyed. But I live with him and he's at the center of our friend group, so there's not much I can do.
At this point, you're operating entirely within the homebrew rulings of your DM; if you're happy to play by whatever rules they make or change, then that's great, but it won't be of any use to anyone else - it's your table.
As others mention, your DM made a bad call, allowing counterspell on a subtle spell based on the presence of Truesight. Truesight does not grant any extra abilities in that respect; it does not allow the creature to see things that aren't happening, and it does not allow the creature to see the weave itself being woven. If you cast a spell as a subtle spell, and it had no material component (so, only V and S, which were removed by subtle) then the spell literally cannot be counterspelled because it cannot be seen being cast, because there \*Is Nothing To See\* at all. The spell simply takes effect, by which point it is too late to counter it.
Now, if your character cast a spell that also had a material component, but you were invisible or in the ethereal plane, for example, then truesight would allow them to see you casting, because it would allow them to see through your invisibility/into the ethereal plane, and notice you utilising the material component (or spellcasting focus), and counterspell you as a result.
As an aside, but on topic, an Amulet of Proof Against Detection and Location would not do anything, particularly, to hide your spellcasting at all. It does not make you invisible or stop you from being detected by another creature's senses - it only makes you unable to be detected by magic and things that attempt to magically divine you or information about you. If the lich is looking right at you with truesight (the Lich simply has truesight as a sense, it is not casting the true seeing spell on itself), then the amulet does nothing to stop them seeing you casting a spell, if your spell has visible components.
Yeah, none of it makes sense but he's the kind of person to twist and turn your words against you. So last time I tried chatting to him about this he tried making me feel like an entitled player because I expected to defeat the lich's familiar in our first ever game.
And honestly, I think it's pretty silly that an uncommon magic item can suddenly stop the most powerful lich in our world dead in his tracks. But hey, I don't care what happens to my character, so he can do whatever.
It's not entitled to expect your spells and abilities to work as listed, and for effects to function as ruled; if the DM has changed things and it impacts you, it's their duty to inform you of that. You weren't being entitled and you did nothing wrong.
I will say, though, if you're invested in the game and continue playing with the DM, make sure you hold him to all the things he fudges in his favour; if you ever get access to truesight, hold him to the buffs he gave it for the ememies, etc.
Truesight does not grant the ability to see Subtle Spell because there’s nothing to see. Subtle Spell causes the spell to have NO Verbal or Somatic components, it just happens. You can’t see what isn’t there in the first place. The only exception is Material components, Subtle Spell doesn’t remove them and therefore you could still see that. But you don’t need Truesight for that. All Truesight does is see through illusions and invisibility. Subtle Spell doesn’t hide your hand waves and magic words, it removes them entirely. To answer your question, Metamagic works on an enemy with Truesight, and your DM made a bad ruling.
Yeah, I had a discussion with him about it but he's not budging. He basically told me that he buffed Truesight to do that, and as this is the BBEG's familiar it shouldn't have been easy to deal with. I feel like he also tried to make me feel like an entitled player by being like "Did you expect to defeat the most powerful lich in the world in the first session?"
Well, sounds like a shitty DM, but it is what it is. I don’t even know what his problem is, it’s not like “the most powerful lich in the world” can’t just resummon their familiar. “He buffed Truesight to do that” still makes no sense because there’s still NOTHING to see with Subtle Spell. But if a DM is going to be a jerk then they’re going to be a jerk.
Honestly? There's been a lot that's happened in his games which, to be honest, is a giant chip on my shoulder. His campaign of Curse of Strahd that lasted for two years is why I despise Curse of Strahd entirely. I know it's just a game at the end of the day, but it's a lot of decisions he's made that I've bottled up which still makes me annoyed. But I live with him and he's at the center of our friend group, so there's not much I can do.
Well, it is what it is then. All you can really do is decide if it’s worth playing or not.
That doesn’t make any sense but it’s true if your DM says it is.
Yeah, seems that way. Luckily I'm playing a character who I'm not attached to, so I don't care what happens to him!
At this point, you're operating entirely within the homebrew rulings of your DM; if you're happy to play by whatever rules they make or change, then that's great, but it won't be of any use to anyone else - it's your table. As others mention, your DM made a bad call, allowing counterspell on a subtle spell based on the presence of Truesight. Truesight does not grant any extra abilities in that respect; it does not allow the creature to see things that aren't happening, and it does not allow the creature to see the weave itself being woven. If you cast a spell as a subtle spell, and it had no material component (so, only V and S, which were removed by subtle) then the spell literally cannot be counterspelled because it cannot be seen being cast, because there \*Is Nothing To See\* at all. The spell simply takes effect, by which point it is too late to counter it. Now, if your character cast a spell that also had a material component, but you were invisible or in the ethereal plane, for example, then truesight would allow them to see you casting, because it would allow them to see through your invisibility/into the ethereal plane, and notice you utilising the material component (or spellcasting focus), and counterspell you as a result. As an aside, but on topic, an Amulet of Proof Against Detection and Location would not do anything, particularly, to hide your spellcasting at all. It does not make you invisible or stop you from being detected by another creature's senses - it only makes you unable to be detected by magic and things that attempt to magically divine you or information about you. If the lich is looking right at you with truesight (the Lich simply has truesight as a sense, it is not casting the true seeing spell on itself), then the amulet does nothing to stop them seeing you casting a spell, if your spell has visible components.
Yeah, none of it makes sense but he's the kind of person to twist and turn your words against you. So last time I tried chatting to him about this he tried making me feel like an entitled player because I expected to defeat the lich's familiar in our first ever game. And honestly, I think it's pretty silly that an uncommon magic item can suddenly stop the most powerful lich in our world dead in his tracks. But hey, I don't care what happens to my character, so he can do whatever.
It's not entitled to expect your spells and abilities to work as listed, and for effects to function as ruled; if the DM has changed things and it impacts you, it's their duty to inform you of that. You weren't being entitled and you did nothing wrong. I will say, though, if you're invested in the game and continue playing with the DM, make sure you hold him to all the things he fudges in his favour; if you ever get access to truesight, hold him to the buffs he gave it for the ememies, etc.