Merap (Mbraa/Mpraa) is a language spoken by a small community in North Kalimantan, Indonesia. It is remarkable in its striking phonology, despite being part of the fairly conservative Kayanic group of languages. This paper describes both the synchronic and diachronic phonology of Merap, as spoken in the village of Langap along the upper Malinau River. It is argued that Merap, despite being highly innovative, is not only a Kayanic language, but that it subgroups specifically with Ngorek, a phonologically conservative Kayanic language of Sarawak and North Kalimantan. The arguments for subgrouping Ngorek and Merap together apart from all other Kayanic languages are purely phonological, as both languages devoiced *-b, but nasalized *-d, a combination found nowhere else in Borneo. Other evidence involves reflexes of nasal-obstruent clusters *mb, *nd, *nj, and *ŋg, and a small list of exclusively shared lexical replacement innovations. Merap is only one of several languages in the area of central Borneo, south of Sabah and north of the equator, that have independently undergone drastic changes. This paper also discusses some of the other phonologically aberrant languages of this linguistic area, including Sa'ban, Modang, Gaai, Kelai, Kiput, and Berawan. Although many of these languages have undergone sound changes that are quite similar to those found in Merap, they are the product of convergence rather than inheritance. Many of the languages in this area have independently shifted stress to the final syllable and expanded the inherited Proto-Malayo-Polynesian vowel inventory.