I agree with Lyya’s math given the mechanics stated above.
Because math and spreadsheets are fun, here’s another set of numbers that y’all might find interesting.
Assuming no cleanse spells, no “restamping” the death mark, and the battle actually lasting another 10 turns, here are the sorted probabilities of each outcome of death mark on a single troop:
Chance Outcome
17.50% Troop is auto-cleansed on turn 3
16.20% Troop is auto-cleansed on turn 2
14.70% Troop is auto-cleansed on turn 4
10.00% Troop is auto-cleansed on turn 1
9.92% Troop is auto-cleansed on turn 5
9.00% Troop dies on turn 1
6.48% Troop dies on turn 2
5.36% Troop is auto-cleansed on turn 6
4.08% Troop dies on turn 3
2.25% Troop is auto-cleansed on turn 7
2.20% Troop dies on turn 4
0.99% Troop dies on turn 5
0.69% Troop is auto-cleansed on turn 8
0.36% Troop dies on turn 6
0.14% Troop is auto-cleansed on turn 9
0.10% Troop dies on turn 7
0.02% Troop dies on turn 8
0.01% Troop is auto-cleansed on turn 10
<0.005% Troop dies on turn 9
I can share the spreadsheet if anyone wants it.