Spoke key.

[Punkawallah]

Yes, you complete and total pervert

M blushes: ‘Fie! Sir! Thou wilt turn mine head!’

 

[Ajax Bay]

On the GCN videos they hold the middle of the spoke with a device but say pliers will do if you don’t have a spoke holder.

Your spokes (Hunt 4S, 24/28 F/R) are 'round' not bladed (which the 'aero' GCN marketeers 'all' use, often 'straight pull', no 'j' bend at hub). Bladed spokes need to stay oriented. Spokes may twist during truing, but once you're done a good squeeze all round will release any twists. Lubrication has been mentioned. This is worth it: one drip on the hub side of the rim, hopefully eyeletted, on all the front spoke holes and the drive side (DS) rear. The reason for not using any on the NDS ones is because they are typically 2/3rds the DS tension, so far less friction (nipple/eyelet).

". . . rare on an old wheel when a rim is out of true only at one point" = I know you say it’s rare on one spot but it’s a Hunt four season and it’s massive in one place. Prob from dropping off the kerb thinking they’re bombproof.

I have 'literally' had to solve a wheel/rim like yours. My son hit a pothole hard resulting in a dented (rim-brake) rim - amazingly he didn't suffer a snake bite (Day 3 of a trek to Cornwall).
I could have sorted out the braking surface but the catastrophe was a significant 'flat' 5cm there to the extent that the NDS spoke had gone loose (so also a local 'buckle' to the right).
It's very difficult (as in only experts on here will say they could) to true a rim radially in this circumstance (which I suspect is yours as well as laterally). LBS, new rim, ERD 10mm more, perchance I had two sets of 16 black spokes right length, new nipples, lace the hub to it, trued on an empty frame. Roll on.
(What’s a good value spoke key? Spokey for me too.)