Ritual of Rebirth

[Agema]

From my experience, you get a random hero of your nation from the HoF back from the dead. I don't know whether it counts ex-members of the Hall of Fame, i.e. ones that have gained heroic abilities but since been knocked down the list.

The mummy is an adequate thug chassis, but I think the best use is reanimating heroes with good magic paths. For instance, some national heroes give you magic paths you don't otherwise have on your national mages, and they can be well worth recovering. Be warned that mummies also have a disease cloud, so don't leave them near disease-vulnerable units on the battlefield, especially valuable ones.