Hi and Welcome.
This dream could be a manifestation of a few things. One, the fear of death? Were you close to your grandfather? Did he offer you security somehow--financially, family, etc. ? His death meant the end of that security.
The coffins with the open lids could represent something in the sub-conscious you wish to bury but is poking its head to the surface, which causes the anxiety.
And finally, the part where the bodies are in "gruesome shape" could be how you feel about this thing from your past you are trying to bury. Does it related to something that was "smashed" such as a hope or dream?
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