Direct:
* Precarious: (adj.) Uncertain, risky, dependent on chance
* Preceptor: (n.) A teacher, instructor, or tutor
* Precinct: (n.) A district or area, often a police jurisdiction
* Precious: (adj.) Valuable, beloved, highly esteemed
Derived:
* Apprehend: (v.) To understand, grasp mentally, or arrest
* Imprecation: (n.) A curse or a prayer for harm
* Intercede: (v.) To plead on behalf of someone else
* Supplication: (n.) A humble request or prayer
* Imprecatory: (adj.) Relating to a curse or imprecation
Note: Some words have different meanings and don't directly relate to "prayer," but still carry the meaning of asking or requesting:
* Preciocity: (n.) High price, value, or cost
* Precocious: (adj.) Developing or maturing at an early age
These are just a few examples, and there are many other words with the Latin root "prec" out there!