This must qualify as the weirdest organism I’ve found in my garden and likely to give me nightmares, so you are warned! A snail and bird parasite that develops in the snail’s tentacles, mimicking a caterpillar by pulsating. I had never heard of such a thing and wondered what the hell was going on, some kind of weird snail sex or maybe some psychedelic mushroom had got into the dinner! The snail is an amber snail / storravsnegl (Succinea putris).
An explanation of the part of the life cycle of the green-banded broodsac or L⁠eucochloridium paradoxum I had witnessed can be read below the video and pictures.

The following is copied from https://animaldiversity.org/accounts/Leucochloridium_paradoxum
“Two visible changes in the snail occur as a result of the accumulation of sporocysts. First, the snail’s tentacles enlarge and pulse in vivid colors. Secondly, the instinctively photophobic snail becomes photophylic and climbs to the tops of trees and grasses. This conspicuous sight looks like a caterpillar to passing birds. The birds swoop down and consume the snail and unknowingly inoculate themselves with Leucochloridium paradoxum. The worm continues its development within the bird’s gut and ultimately ends up in the rectum where its eggs are flushed out with waste. Another snail consumes the excreted egg and the cycle continues”