Not too easy to do in iClone. You need to reduce the opacity of every part of the avatar - upper, lower, hair, skin etc., etc.
You can also do the dissolve down in an NLE if you export one scene complete with the beam, but minus the avatar and a second scene with only the avatar on a green or blue background. Lay the avatar clip over the main scene clip and dissolve the avatar out when needed.
If the avatar is inside the beam then in order for everything to look right you need a main scene clip minus beam and avatar. A green/blue screen scene with the fading beam and a green/blue scene with just the avatar.
Place the main scene on one video track, add the avatar clip above this and then place the beam on top. Key out the green/blue backgrounds and fade the avatar track out in time with the beam fade.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.