One of the book's most significant contributions is its extensive appendix for integrating Lovecraftian horror into standard D&D campaigns. It provides:
: The system adapts classic feats for a modern or 1920s setting—transforming "Ride-By Attack" into "Drive-By Attack"—while prioritizing investigative skills over pure combat prowess. Bridging Two Worlds Call of Cthulhu - D20 Core
: Stats for entities like Cthulhu and Nyarlathotep designed to challenge high-level d20 parties. One of the book's most significant contributions is
Critics and fans often view Call of Cthulhu d20 as a polarizing but influential experiment. While some purists feel the d20 system is inherently too "heroic" for cosmic horror, others praise the book for its exceptional writing and the accessibility it provided to players already familiar with d20 mechanics. Today, it remains a sought-after collector’s item and a valuable resource for GMs looking to inject psychological horror into their modern or fantasy games. Critics and fans often view Call of Cthulhu
: Unlike the hit-point cushions found in other d20 titles, this edition features "Massive Damage" thresholds that make even low-level threats potentially fatal.