I couldn’t find any information about the whereabouts of these items, so this guide might come in handy.
Locations of Useful Items
The helmet allows you to additionally withstand a few more blows.
You can find it after the first exit to the surface. Having reached the ruined building – press use to go down the stairs. In the cave, you can pick up a helmet from a corpse.
The chain allows you not to drop the sword from your hands after blocking strong blows and unsuccessful rolls into the wall.
Shortly after finding the helmet, you will again go underground. After reaching the room in which you need to roll to lower the elevator, continue to the left and climb up.
In the next cave, in a pile of bones, you will find a chain.