You can never have enough undead wandering the halls of your dungeons. The tomb guardian and boneshard have been converted to 5e so that your adventurers may continue to fear skeletons once they are past level 5. You're welcome.
Tomb guardians have four weapons, and consequently can make four attacks. Their quick reflexes and high base speed ensure that your players will only escape with difficulty. The creature should make a good boss for a mid level group.
Boneshards are suicide troops (more so, at least, than other skeletons). A single boneshard would make a great encounter, although they should be used with caution, as they could possible oneshot lower HP characters. When they are first introduced to the players, you should indicate that the skeletons appear "unstable". Perhaps they twitch with energy.
You can download a pdf of the conversions here.