Hello everyone. I felt the need to do this since unlike Runes, Relics get new additions once in a while, and it'd be a chore to update every single one of my analyses just to add said Relic to my list of options. Instead, I will analyse every respective Relic here and tell in which situations or on which kind of monster they'd be useful on so that you may experiment more freely and I may relieve myself of the need for updating my past analyses.
Before beginning, I must mention that some Relics are obviously weaker than others and some Relics are not useful at all. To more effectively show this, I will list the higher powered Relics before those inferior ones so that you can know the one below is weaker than the one above, but can sometimes be used as a substitute if you don't have access to the latter.
Swords are offensive Relics that focus on dealing damage. However, there are some that also have survival and control effects.

Jakugan Sword: This is the simplest Sword: if the enemy's Life is below 50% after hitting, they take even more damage. This also applies to AOEs, so you can use a powerful AOE to be then boosted by Jakugan's Sword. This is a generally useful Relic that will serve well if you're going for some kind of OTK or just to put more juice into attackers in general.

Nishant's Sword: This is more situational than Jakugan Sword, but in the case that it does proc, it has a very strong effect. It does the same damage as Jakugan Sword, but it also drains Stamina from the enemy - naturally, it applies to AOEs as well, making Nishant's Sword useful for Stamina Drain strategies or just to have a more powerful Jakugan Sword. Keep in mind that control-like Swords are extremely rare, which adds to Nishant's Sword's usefulness.

Solaris' Sword: This is Jakugan's Sword, but reversed in terms of Life requirements. It's a bit counterproductive though, because if you want to go for raw damage, you want to dish out great damage as well, which would involve dealing more than 50%.

Cain's Sword: One of the few Relics that have 2 effects, Cain's Sword is a strictly better Jakugan's Sword. It does the same damage at the same Life range, but it has another effect as mentioned before: if the wearer's Life is below 50% after hitting the enemy, the wearer heals by a great amount. An excellent Relic for both damage output and survival that's even more splashable than Jakugan Sword.

Soul-Shattering Greatsword: This can be very useful on Stunners/Possessors that have Sword slots, or on any monster with AOE versions of those effects in general. It can help a lot with Stamina Drain strategies, especially if you have Wangzhou on your team.

Ice-Breaking Greatsword: If you favor survival and don't have Cain's Sword, this can be a good alternative on monsters with Freeze/Stun effects. Plus, unlike the previous Sword, it doesn't need to be an AOE to have its full effect.

Earth-Shaking Sword and Ice-Breaking Sword: These are damage dealing versions of the last two Swords, but they sadly aren't too impressive. You can use them if you don't have any of the above.

Soul-Shattering Sword: Inferior Soul-Shattering Greatsword.

Punisher Sword: Skip.

Avenger Sword: Skip.

Darkness Sword: Skip.

Flaming Sword: Skip.

Exhausting Sword: Not as bad as the last four, but it's still pretty bad.