(Capcom's primary storyline)
Queen Plaga was a large plant-like organism which was the "mother" of the Plaga.
The Queen Plaga was an organism based in a
castle tower. It was important to Los Iluminados, and it was here where new Plaga parasites were produced.  In preparation for [excerpt 1] Ashley Graham's indoctrination into the cult, a ritual was performed here under the watch of Castellan Ramon Salazar.
In a desperate decision to kill
Leon S. Kennedy, Salazar allowed himself and his remaining Verdugo bodyguard to be assimilated into the organism. However, the new chimera failed in its attack on Leon and all three constituent parts perished.
Excerpt from biohazard4 kaitaishinsho revised edition , page 140: "古城の城主サラザールが、側近とともに寄生生物の巨大な母体と融合した姿。 中心から生えたシェルター内にサラザール本体が存左し、３本の長い触手を振りまわして敵を攻撃する。 なかでも、先端に側近の顔が浮かんだ中央の触手は、恐るべき殺傷力を秘めた牙を持つ。 また、 根元には胞子状の寄生体が無数に植えつけられており、そこからつぎつぎとプラーガを生み出す。"
↑ Hamamura (ed.),
biohazard4 kaitaishinsho revised edition, p.140.