After you return to Sigil from the Planes you will be able to find Greater Glabrezu in UnderSigil, the "dungeon" under the Clerk's Ward. These tough monsters are worth 70,000 experience points each, and when you kill them they drop either a large amount of money or a powerful item.
I ran some tests to figure out exactly how likely you are to find a Greater Glabrezu each time you go into UnderSigil, where they might spawn and what chances you have of finding each item they can drop.
Run 3Item | Number of GG to have item |
Percentage of GG to have item |
Copper Commons | 857 / 1000 | 85.7% |
Aegis of Torment | 46 / 1000 | 4.6% |
Bell's Shield | 42 / 1000 | 4.2% |
Ring Zero | 21 / 1000 | 2.1% |
Umei Kaihen | 19 / 1000 | 1.9% |
Heaven and Earth | 15 / 1000 | 1.5% |
Actually finding a Greater Glabrezu in UnderSigil can be difficult. Sometimes you can go down there ten times in a row without finding one, other times you may find two at once. To test just how likely it is that a Greater Glabrezu will be spawned each time you enter UnderSigil I made a script to simulate entering it 1000 times and keep track of how many Greater Glabrezu should have spawned each time.
Number
of GGNumber of runs to
have that many GGPercentage of runs to
have that many GG
0 GG
663 / 1000
66.3%
1 GG
274 / 1000
27.4%
2 GG
58 / 1000
5.8%
3 GG
5 / 1000
0.5%
4 GG
0 / 1000
0%
5 GG
0 / 1000
0%
6 GG
0 / 1000
0%
This is a map of UnderSigil with all the possible Greater Glabrezu spawn points on it. The three different colors indicate that if you find a Greater Glabrezu at one point you should look for more at the other points of the same color and not bother checking the other colored points during that run in UnderSigil. Finding a Greater Glabrezu at one point does not guarantee that there will be a Greater Glabrezu at the other points of the same color, but simply that those will be the only other possible points during that run for a Greater Glabrezu to spawn.