Hard coating protects little one from spawn of the underworld? (8)

I believe the answer is:

eggshell

I'm a little stuck... Click here to teach me more about this clue!

'spawn of the underworld?' is the definition.
'eggshell' can be an answer for 'spawn' (thesaurus). I am not sure about the 'of the underworld?' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for eggshell that I've seen before include "Thin type of china" , "formed in layers" , "Slightly glossy (of paint)" , "Sort of paint" , "cracker?" .)

I've seen this clue in The Irish Times.

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