Hrodulf house is one of the many interesting locations you can explore in Skyrim: Dragonborn.
You can find Hrodulf's House southeast of Raven Rock, and north east of the shipwreck, Wreck of the Strident Squall. You are most likely to come across this house when doing the quest "March of the Dead", since Hrodulf's House is located in between the Old Attius Farm and Fort Frostmoth.
At first sight, the house appears like just some random abandoned structure with nothing interesting to see. However, it hides a big secret area underneath, with more loot and treasure.OUTSIDE THE HOUSE
As you approach Hrodulfs House, you will get attacked by some of those Ash Spawn, so be prepared for combat
Once you have beaten the Ash Spawn, start by exploring what's outside of the house. There are Heart Stone Veins here. You can mine them to get a Heartstone. Heart Stones are used for anchoring an Ash Guardian without it attacking you. The Heart Stone can also be sold to Neloth.INSIDE THE HOUSE
Next, enter the house. Loot any items and chests you find in the main level. But what you are really looking for, is a trapdoor that will lead you into a new secret area underground. Find the trapdoor (basement door), open it, and go down underneath the houseUNDERGROUND AREA
Here you will find some more loot, but there is yet another hidden area that you can discover. Look for a bookshelf, and interact with the bookshelf to reveal a secret tunnel behind it. Go through the tunnel and find a dead body with Hrodulf's Journal on it. According to the journal, Hrodulf heard some strange noises from underneath his house and went to investigate and apparently died while doing so. The journal ends with the creepy message, "Fire from the Deep"
For more info on Hrodulf's House including where to find all the secret doors, see the wiki:Skyrim Dragonborn: Hrodulf's House