The nether reactor gives friendly mobs (players, tamed mobs, villagers, etc.) fire resistance, gives striders Speed III, and clears Wither I in a 64 block radius. It is activated using a complex structure shown on the next page.
In order to form a full reactor, you make the above multiblock structure, building from layer 1 upwards.
It is made using lots of valuables, including a nether star… good luck!