Kali Yantra along with its Mool Mantra is considered very effective medium to fulfil Kali Sadhana.
ॐ क्रीं क्रीं क्रीं हूँ हूँ ह्रीं ह्रीं दक्षिणे कालिके
क्रीं क्रीं क्रीं हूँ हूँ ह्रीं ह्रीं स्वाहा॥
Om Kreem Kreem Kreem Hum Hum Hreem Hreem Dakshine Kalike
Kreem Kreem Kreem Hum Hum Hreem Hreem Svaha॥