The Inner Controller
Niyantṛ (controller, regulator) describes one of the most important aspects of Brahman's relationship to all beings and matter. Rāmānuja, following the Bṛhadāraṇyaka Upaniṣad's antaryāmin passages, establishes that the Lord pervades and controls all beings from within — yet they do not know Him.
The Antaryāmin Brāhmaṇa
The classic scriptural basis is the Antaryāmin Brāhmaṇa of the Bṛhadāraṇyaka Upaniṣad, where Yājñavalkya teaches that the one who dwells within the earth, within water, within fire — whom these do not know, whose body these are, who controls these from within — is the inner controller, the immortal (antaryāmī amṛtaḥ).
Central to Sarīra-Sarīrī-Bhāva
The niyantṛ doctrine is central to the body-soul relationship of Vishishtadvaita. If Brahman is the soul (sarīrī) and all beings are His body (sarīra), then Brahman naturally controls this body from within, just as a human soul controls its body. Niyantṛtva (the capacity to control) is thus integral to Brahman's being the inner self of all.
Free Will Reconciled
Rāmānuja reconciles divine control with human freedom: the Lord is the ultimate cause and enabler of all action, but He acts in accordance with the jīva's own karma and choices. The niyantṛ does not override the jīva's capacity for choice but is the substratum that makes the capacity for action possible at all.