When Ryan dived into his mother's nightmare he couldn't have expected an army of shadows trying to completely obliterate his soul which was not being protected by his body.
To understand this we need to know that the body is actually a vessel for one's soul and also protects it from direct damage. Now when a being enters someone else's dream they need to leave their body outside which makes them prone to damage Inside dreams.
For some reason these shadows were capable of damaging his soul to a frightening degree so had to measure each and every movement of his body.
Ryan looked at the approaching army of shadows as he muttered "Boogeyman." Unfortunately, the armor didn't cover his soul as it did with his body.
The only reason he could summon Umbra was that she was a weapon that was quite similar to soul weapons. She would always remain in his soul and not in his body so this gave him the liberty to chip away large units of shadows at once.
