Directly quoting from this question on HSE itself, which in short, (kind of) gives the answer to your question:
Although Vimsottari is popular and more applicable daśā (दशा ) among all,
there are still may other type of dasa.
Some of them are Astottari, Shodsottari, Dwadasottari, Panchottari,
Satabdika, Chaturshiti-sama, Dwisaptati-sama, Shastibayani,
Shatvinsa-sama, Kalachakra, Chakra, Chara, Sthira, Kendra,
Brahmagraha, Mandook, Yogardha, Shoola, Drig, Trikona, Rasi,
Panchaswara, Yogini, Naisargik, Pinda, Sandhya, Ashtavarga, Pachaka,
[The above list has been taken from the Brihat Parashara Hora Shastra.]
Brahmagraha (ब्रह्मग्रह ) is an astrology -Jyotiṣa Śāstra (ज्योतिष शास्त्र ) related term, which relates to the graha daśā (ग्रह दशा), which is formed as a result of certain planetary combinations in the Janmakuṇḍalī of the jātaka (whose Kuṇḍalī is being analysed).
Brahma Graha is explained in precise details in this excerpt from Chapter 46. Dashas of Grahas of the Brihat Parashara Hora Shastra:
170-173. Maitreya said. O great Sage! Now please enlighten me
how the Brahm Graha is picked out in a chart. Maharishi
Parashar said. From amongst the Lords of Ari, Randhr and
Vyaya the Grah, who may possess the greatest strength and is
placed in Lagn, or in Yuvati with strength, the one, placed
in an odd Rāśi within sixth Bhava from the Bhava concerned,
is called the Brahm Grah. The Lord of Randhr in Randhr is
also accepted, as Brahm Grah. If Śani, or Rahu/Ketu obtain
Brahmatva (qualifications of Brahm Grah), they become Brahm
Grah. If a number of Grahas obtain Brahmatva, the one with
the largest number of degrees would become Brahm Grah. If
there is parity in the degrees of such Grahas, the most
powerful amongst them would become Brahm Grah.
Furthermore, this, article based on Gemini Astrology discuss the concept in more detail, albeit in a different way or methodology per say, to calculate and find various kinds of Brahma Graha combinations.
Another article, from a contemporary ayurveda history book based on Atharva Veda, delineates Brahmagraha as a class of evil demons or spirits, which caused some ailments or diseases. Basically, representing the disease causing agents in a personified form of evil spirits or demons.
The Book 2, Verse 9 of the Atharva Veda also make some reference to this as a disease and alludes to some charms and rituals to mitigate it and relieve the afflicted from its malefic effects.
The english translation by Griffith for the same verse, however, doesn't mention any Brahma graha specifically.
Brahma Graha is a planet calculated as per relevant methods of astrology, to understand the malefic effect of the planetary daśās vis-a vis the so called - Sthira daśās ( स्थिर दशा ) and the Māraka daśās ( मारक दशा ) in a horoscope.
Or, Brahma Graha is an evil spirit or demon which causes ailments and diseases.