فَأَنذَرتُكُم نارًا تَلَظّىٰ

So I warn you of a blazing fire.

EXEGESIS

The verb talaẓẓā comes from the noun laẓā which means a pure flame, as per 70:15. A pure flame is one that does not have smoke and thus has a higher temperature.^{‎[1]Amthal, 20/260.}

According to Ṭūsī, laẓā is also one of the specific names of the fire of hell.^{‎[2]Tibyan, 10/365.}

The word nār (fire) has come in an indefinite form and this is a rhetorical tool to increase reverence and fear.^{‎[3]Munyah, 30/409.}

EXPOSITION

This verse is a complement to verse 12 which stated that guidance is upon Allah. There are two facets to guidance: one is giving glad tidings to those who follow the guidance and the other is warning those who refuse to accept it. When Allah appoints prophets, He gives them these two responsibilities, for merely describing the right path without giving any motivating factor (glad tidings) or deterrent (warning) does not effectively bring people towards the right path, except those who are at a higher intellectual level and are able to accept truth without any other considerations.

This verse, therefore, is an instance of warning and threatening.

[1] Amthal, 20/260.
[2] Tibyan, 10/365.
[3] Munyah, 30/409.