In Magos' Warcraft 3 Model Editor, go to "Windows" then "Geoset Animations" right-click in the white blank then click "Create New". In Geoset ID select the Geoset you want to disappear then under Alpha click "Animated" then click the once greyed out "Alpha" there you should type the animation number and the visibility. For example the death animation is 333 then type there 333: 0. This way when the animation that starts with frame 333 is played, the selected geoset will disappear. To know the number of Animation, just click "Windows" then "Sequence Manager".