You'll be looking for a axial lock nut socket. You'll have to determine the size nut. They have a numerical designation, smaller nuts have lower numbers and larger nuts have bigger numbers. You can find the dimensions in a Machinists Handbook, metric and sae. If you can't find a Machinists Handbook but can get some dimensions from the nut I can look it up in my copy of the handbook.
How much shaft is sticking out of the nut? You may need a deep well version.
Confidence: The feeling you experience before you fully understand the situation