In Horoscope reading you have two types of malefic, one is the Natural Malefic like Sun , Saturn and Mars , then you have functional malefic. Functional malefic are those planet whose Mooltrikona sign is in the trik bahavas that is the 6th, 8th and the 12th house. For Tula Lagan if you want to know you have Rahu & Ketu as Malefic and also Mercury because it is the 12th lord.
If you browse the AstrologyMag.com website, you will find many articles which are highly informative.
For start you can read more about Rahu and Ketu in this article on Rahu and Ketu the Spiritual Purifier
Blessings,
Navneet Khanna
Astrologer