One of the very common Wikidata task is item merging which in most parts works great. Unfortunately some fraction of the merges are wrong, for whatever reasons: cluelessness, vandalism, hasty merges, etc. I see a lot of merges of similar paintings, which have the same title and author, merges of different people with same dates of birth and death, merges of items for portrait of a person with item for a person. etc.

We need a tool for easy un-merge. Right now un-merging involves:

1. Restoring merged item to the previous version
2. Restoring redirect to the previous version
3. The third step should be review of all the links to the new item to figure out which item to assign it to. This step is especially painful.

The unmerge tool should do all those 3 tasks (are there more ?) and we should probably have a link to the undo in the history listing of both items.