From publisher blurb:
Dark Places & Demogorgons for Old-School Essentials and Dark Places & Demogorgons The Cryptid Manual, utilizes the B/X rules revised to perfection in Old-School Essentials by Gavin Norman of Necrotic Gnome. Dark Places & Demogorgons for Old-School Essentials and The Cryptid Manual will give the ultimate 1980s Retro Roleplaying experience.
Does this require Old-School Essentials to play?
Yes. Nothing has been doubled up from Necrotic Gnome's Old-School Essentials, including any material from the Player Characters section of OSE (e.g. including the rules for character creation and ability scores) or the Adventuring section of OSE (e.g. including the rules for combat, saving throws, ability checks, dungeon exploration, etc.). With Dark Places & Demogorgons for OSE, you're getting all new gaming material.
Dark Places & Demogorgons The Cryptid Manual for OSE will include the cryptid monsters, each fully detailed with descriptions and stats.