Eskêl is the language spoken in most of Covasna County, eastern Harghita County, around Bicaz in Neamț county, and western Bacău County *here*, by the Eskêl (associated with Szekelys and Szekely Csangos *there*) people descended from Avars, Bulgars, Pechenegs and Cumans (with elements of Hussite refugees and Transylvanian Saxons), with a Catholic majority and Calvinist minority (~400,000 speakers). Many of Eskêl people in Maros and Aranyos seats had emigrated to Moldavia. The language has 3 major dialect groups, i.e. Eastern (Csik), Western (Udvarhely) and Southern (Haromszék); the Hétfalu subdialects in Braşov County is a subdialect of southern dialect; in Moldavia, the Szekely Csangos traditionally speak a mixed dialect, originated from Maros and Udvarhely regions and can be classified as a western dialect.
The language is influenced by Latin, Church Slavonic and Middle High German, as well as Kipchak and Oghuz branches of Turkic; Hungarian and German being the main source of transmitted loanwords. Due to its isolation in the mountains, the language was able to preserve many archaic features, while language contact used to be pervasive.
PS: there is no consensus about the origin of Szekelys, a Turkic hypothesis relates it to the Turkic tribal name Esegel/Eskil and the language and speaker's name can also be derived from it; meanwhile other peoples, including Magyars and Romanians, migrating into this area got assimilated to this language. Cf. the Wikipedia entry of Esegel:
Among other writers who mentioned Esegels, the Persian “Geography“ of 982 named Ishkils as one of three Bulgarian tribes, who were constantly conflicting among themselves. Gardizi, the author of the composition Zain al-ahbar (mid-11th century), wrote: "Between possessions of Bulgars and possessions of Eskels, who also belong to Bulgars, is a Magyar area. These Magyars are also a Türkic tribe". Constantine Porphyrogenitus wrote that endoethnonym of the "Magyar Türks" was Savartoiaskaloi, i.e. Savart (Suvar/Sabir) and Eskel. Yu.A.Zuev summarized that It is held that Eskels (Esegels) merged with Hungarians (Magyars). The ethnographic group Sekei (written form Szekler) are recognized as their descendants. Esegels left a prominent trace in the Northern and Western Europe as last and first names, mostly in the form "Askel".