If you’re getting an error similar to “Can’t update: signature from *** is marginal trust” or “invalid or corrupted package” you probably just need to update your package signing keys:
sudo pacman -Sy archlinux-keyring endeavouros-keyring
sudo pacman -Syu
Also make sure your system clock is correct!
Of course, it’s also possible that the package file actually is corrupt. Remove it from /var/cache/pacman/pkg/ so pacman will download it again.
If no package file is explicitly mentioned then you may have an incomplete download. Run sudo rm /var/cache/pacman/pkg/*.part to remove any partial downloads.
If you have any other errors, you can clear out the pacman keyring and start fresh:
sudo mv /etc/pacman.d/gnupg /root/pacman-key.bak
sudo pacman-key --init
sudo pacman-key --populate archlinux endeavouros
sudo pacman -Syy archlinux-keyring endeavouros-keyring
sudo pacman -Syyu
And if this still doesn’t work, and you trust that the packages are actually correct and not corrupt and haven’t been interfered with, then you can force (re)installation of the keyring packages:
sudo pacman -U /var/cache/pacman/pkg/{archlinux,endeavouros}-keyring*.pkg.tar.zst
then try again.
If you’re installing an AUR package a PGP key can be used to verify the source files. You will need to import this into your personal keyring before it can be verified. If you don’t you’ll get an error similar to:
[...]
llvm-5.0.0.src.tar.xz ... FAILED (unknown public key 0FC3042E345AD05D)
libcxx-5.0.0.src.tar.xz ... FAILED (unknown public key 0FC3042E345AD05D)
libcxxabi-5.0.0.src.tar.xz ... FAILED (unknown public key 0FC3042E345AD05D)
[...]
To “fix” this, simply import the key:
gpg --recv-key 0FC3042E345AD05D
Fixed things perfectly. 
