Messier 54 is a globular cluster in the constellation Sagittarius that was discovered by Charles Messier in 1788 [1] in his valiant efforts to identify every fuzzy thing in the sky that wasn't a comet and therefore could be ignored because, from Messier's perspective, the only interesting things in astronomy are comets. Like other globular clusters, Messier 54 looks like a giant sphere of stars. The mass of Messier 54 is 1.41 million times the mass of the Sun [2], which means that the cluster could be thought of as containing approximately 1.41 million stars. The cluster also has an apparent diameter as seen from Earth that is roughly one-third the diameter of the moon [3]. This is rather surprising given how far away the globular cluster is. The center of the Milky Way is also located in Sagittarius, and Messier 54 is actually on the far side of that in a place that is about 86 thousand light years (26.3 kpc) from Earth [4].
However, unlike most other globular clusters in the Messier catalogue, Messier 54 does not appear to be part of our galaxy. Instead, it appears to be part of another galaxy called the Sagittarius Dwarf Spheroidal Galaxy [5, 6]. This dwarf galaxy was discovered by accident just like a lot of other things in astronomy. Astronomers found it in 1994 when performing a survey of stars in the galactic bulge [7]. It turned out that some of the stars were not in the bulge but part of another round but very faint and diffuse galaxy on the opposite side of the bulge that no one had spotted before because of how challenging it is to see a faint and diffuse galaxy through the center of our own galaxy.
I'm not going to talk too much about the Sagittarius Dwarf Spheroidal Galaxy itself because I want to focus on Messier 54. However, the important thing to keep in mind is that the dwarf galaxy is, like other nearby dwarf galaxies, gravitationally interacting with and falling into the Milky Way, but the difference between the Sagittarius Dwarf Spheroidal Galaxy and most everything else is that the Sagittarius Dwarf Spheroidal Galaxy has already been mostly gravitationally pulled apart. It has apparently done several orbits of the Milky Way already, and people have identified streams of stars torn from the galaxy that loop around the Milky Way several times [8]. It currenty has a mass of 400 million times the mass of the Sun [9], but it used to be about a factor of 100 larger [10].
So back to Messier 54. When people found the Sagittarius Dwarf Spheroidal Galaxy, they identified that it was in the location and at about the same distance as Messier 54 [5, 6], although it looks like the cluster is offset from the center of the dwarf galaxy. People have since identified that Messier 54 could be, in fact, something called a nuclear star cluster. Many other dwarf elliptical galaxies have some sort of small nuclear clusters that have formed in their cores, and Messier 54 appears to be a particularly close version of one of these clusters. The expectation is that, if the Sagittarius Dwarf Spheroidal Galaxy had not been stretched into spaghetti by the Milky Way, the galaxy would have continued to form stars very happily over time and may have even absorbed some additional intergalactic gas, and Messier 54 would have gone on to also grow in size and form a larger core of stars at the center of that galaxy.
One other thing worth noting about Messier 54 is that a lot of people think that it could contain an intermediate mass black hole. Observations of the motions of the stars within the center of Messier 54 that were published in 2009 indicated that such a black hole with a mass 9400 times the mass of the Sun could be present in the cluster's center [11]. This black hole's mass would place it in between the star-sized black holes we seem to find scattered around our galaxy and other galaxies and the supermassive black holes that are millions or billons of times the mass of the Sun that we find in the centers of galaxies, which is why the black hole in Messier 54 is called an intermediate mass black hole.
Astronomers expect that the masses of supermassive black holes in the centers of galaxies scale with the sizes of their stellar bulges if they are spiral galaxies or with the sizes of the galaxies themselves if they are elliptical, so if we think of Messier 54 as being a very small stellar bulge within the Sagittarius Dwarf Spheroidal Galaxy, we might expect it to have a black hole that is also a small version of a supermassive black hole. However, follow-up X-ray observations of Messier 54 failed to identify any emission that would be associated with gas falling into that black hole [12], and no one seems to have performed any observations since 2011 either proving or disproving that a black hole is there.
If you want to find Messier 54 itself, it's relatively straightforward to find, although you too probably won't find much evidence of a black hole. The cluster is located on the bottom of the Teapot in Sagittarius slightly less than 2 degrees away from Zeta Sagittarii (the star where the handle connect to the bottom of the teapot) roughly in the direction of Epsilon Sagittarii (the star where the spout connects to the bottom of the teapot). The cluster has a magnitude of about 8 [13], so it's fainter than what can be seen naked eye but relatively easy to see in even a small telescope or a pair of binoculars. However, the brightest part of the cluster is relatively compact, so it might be mistaken for a star if the magnification is too low or the telescope is too small [1, 14]. Go for a bigger telescope with moderate to high magnification to see the spatial extent of the cluster.