SB 4.24.18

ātmārāmo ’pi yas tv asya
 loka-kalpasya rādhase
śaktyā yukto vicarati
 ghorayā bhagavān bhavaḥ
Word for word: 
ātma-ārāmaḥ — self-satisfied; api — although he is; yaḥ — one who is; tu — but; asya — this; loka — material world; kalpasya — when manifested; rādhase — for the matter of helping its existence; śaktyā — potencies; yuktaḥ — being engaged; vicarati — he acts; ghorayā — very dangerous; bhagavān — His Lordship; bhavaḥ — Śiva.
Translation: 
Lord Śiva, the most powerful demigod, second only to Lord Viṣṇu, is self-sufficient. Although he has nothing to aspire for in the material world, for the benefit of those in the material world he is always busily engaged everywhere and is accompanied by his dangerous energies like the goddess Kālī and the goddess Durgā.
Purport: 

Lord Śiva is known as the greatest devotee of the Supreme Personality of Godhead. He is known as the best of all types of Vaiṣṇavas (vaiṣṇavānāṁ yathā śambhuḥ). Consequently, Lord Śiva has a Vaiṣṇava sampradāya, the disciplic succession known as the Rudra sampradāya. Just as there is a Brahma sampradāya coming directly from Lord Brahmā, the Rudra sampradāya comes directly from Lord Śiva. Lord Śiva is one of the twelve great personalities, as stated in Śrīmad-Bhāgavatam (6.3.20):

svayambhūr nāradaḥ śambhuḥ
 kumāraḥ kapilo manuḥ
prahlādo janako bhīṣmo
 balir vaiyāsakir vayam

These are twelve great authorities in preaching God consciousness. The name Śambhu means Lord Śiva. His disciplic succession is also known as the Viṣṇusvāmi sampradāya, and the current Viṣṇusvāmi sampradāya is also known as the Vallabha sampradāya. The current Brahma sampradāya is known as the Madhva-Gauḍīya sampradāya. Even though Lord Śiva appeared to preach Māyāvāda philosophy, at the end of his pastimes in the form of Śaṅkarācārya he preached the Vaiṣṇava philosophy: bhaja govindaṁ bhaja govindaṁ bhaja govindaṁ mūḍha-mate. He stressed worshiping Lord Kṛṣṇa, or Govinda, three times in this verse and especially warned his followers that they could not possibly achieve deliverance, or mukti, simply by word jugglery and grammatical puzzles. If one is actually serious to attain mukti, he must worship Lord Kṛṣṇa. That is Śrīpāda Śaṅkarācārya’s last instruction.

Herein it is mentioned that Lord Śiva is always accompanied by his material energy (śaktyā ghorayā). Material energy — goddess Durgā, or goddess Kālī — is always under his control. Goddess Kālī and Durgā serve him by killing all the asuras, or demons. Sometimes Kālī becomes so infuriated that she indiscriminately kills all kinds of asuras. There is a popular picture of goddess Kālī in which she wears a garland composed of the heads of the asuras and holds in her left hand a captured head and in her right hand a great khaḍga, or chopper, for killing asuras. Great wars are symbolic representations of Kālī’s devastation of the asuras and are actually conducted by the goddess Kālī:

sṛṣṭi-sthiti-pralaya-sādhana-śaktir ekā

(Brahma-saṁhitā 5.44)

Asuras try to pacify the goddess Kālī, or Durgā, by worshiping her in material opulence, but when the asuras become too intolerable, the goddess Kālī does not discriminate in killing them wholesale. Asuras do not know the secret of the energy of Lord Śiva, and they prefer to worship the goddess Kālī or Durgā or Lord Śiva for material benefit. Due to their demoniac character, they are reluctant to surrender to Lord Kṛṣṇa, as indicated by Bhagavad-gītā (7.15):

na māṁ duṣkṛtino mūḍhāḥ
 prapadyante narādhamāḥ
māyayāpahṛta-jñānā
 āsuraṁ bhāvam āśritāḥ

Lord Śiva’s duty is very dangerous because he has to employ the energy of the goddess Kālī, or Durgā. In another popular picture the goddess Kālī is sometimes seen standing on the prostrate body of Lord Śiva, which indicates that sometimes Lord Śiva has to fall down flat in order to stop the goddess Kālī from killing the asuras. Since Lord Śiva controls the great material energy (the goddess Durgā), worshipers of Lord Śiva attain very opulent positions within this material world. Under Lord Śiva’s direction, a worshiper of Lord Śiva gets all kinds of material facilities. In contrast, a Vaiṣṇava, or worshiper of Lord Viṣṇu, gradually becomes poorer in material possessions because Lord Viṣṇu does not trick His devotees into becoming materially entangled by possessions. Lord Viṣṇu gives His devotees intelligence from within, as stated in Bhagavad-gītā (10.10):

teṣāṁ satata-yuktānāṁ
 bhajatāṁ prīti-pūrvakam
dadāmi buddhi-yogaṁ taṁ
 yena mām upayānti te

“To those who are constantly devoted and worship Me with love, I give the understanding by which they can come to Me.”

Thus Lord Viṣṇu gives intelligence to His devotee so that the devotee can make progress on the path back home, back to Godhead. Since a devotee has nothing to do with any kind of material possession, he does not come under the control of the goddess Kālī, or the goddess Durgā.

Lord Śiva is also in charge of the tamo-guṇa, or the mode of ignorance in this material world. His potency, the goddess Durgā, is described as keeping all living entities in the darkness of ignorance (yā devī sarva-bhūteṣu nidra-rūpaṁ saṁsthitā). Both Lord Brahmā and Lord Śiva are incarnations of Lord Viṣṇu, but Lord Brahmā is in charge of the creation whereas Lord Śiva is in charge of the destruction, which he carries out with the help of his material energy, goddess Kālī, or goddess Durgā. Thus in this verse Lord Śiva is described as being accompanied by dangerous potencies (śaktyā ghorayā), and that is the actual position of Lord Śiva.