the unnamed , easily substitutable forces that serve as whatever terror the heroes of horror movies try to avoid 
(ROOT (NP (NP (DT the) (JJ unnamed) (, ,) (ADJP (RB easily) (JJ substitutable)) (NNS forces)) (SBAR (WHNP (WDT that)) (S (VP (VBP serve) (PP (IN as) (SBAR (WHNP (WDT whatever) (NN terror)) (S (NP (NP (DT the) (NNS heroes)) (PP (IN of) (NP (NN horror) (NNS movies)))) (VP (VBP try) (S (VP (TO to) (VP (VB avoid)))))))))))))
(ROOT (NP (NP (DT ) (JJ ) (, ) (ADJP (RB ) (JJ )) (NNS )) (SBAR (WHNP (WDT )) (S (VP (VBP ) (PP (IN ) (SBAR (WHNP (WDT ) (NN )) (S (NP (NP (DT ) (NNS )) (PP (IN ) (NP (NN ) (NNS )))) (VP (VBP ) (S (VP (TO ) (VP (VB )))))))))))))