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Yesterday, I was testing the alternate upgrade for the Dark Champion.
I want to say right away that I'm not insisting on anything; I'm just going to share my opinion and impressions.
So, I think the "deadly strike" ability should be replaced. It really creates an imbalance. Out of eight attacks, the unit is guaranteed to deal two deadly strikes, but it's never known when it will happen. Whether it will be during an attack or a counterattack is unknown to either the unit's owner or its opponent. In other words, the ability introduces not so much an imbalance as a certain amount of chaos and a great dependence on randomness, because neither the player controlling the undead nor their opponent can know exactly how, on whom, and when this ability will work, which means both are at risk, and it can ruin the tactics for both. This situation doesn't occur with any other unit. The only unit that still has this ability is a neutral unit that is encountered rarely and in small numbers.
For example, yesterday I had a case where I attacked with 18 Phantom Riders against almost 30 Archangels, and the "deadly strike" ability worked on the very first attack. As a result, -15 Archangels immediately. I got lucky. But such luck is too random and unpredictable.
By the way, it's also important to remember the "Soldier's Luck" skill, which increases the probability of units using their abilities.
In addition, it should be remembered that the effectiveness of the ability increases proportionally with the number of units. That is, when two such dragons fight against, for example, four Cyclops, the deadly strike may not be very noticeable, but when, for example, 25 dragons fight against 40 Cyclops and deal -20 Cyclops in one turn, that's significant.
I'm still in favor of giving this unit abilities related to weakening, such as fear, sorrow, weakening, etc. The original dragon, I think, had an ability where, when it attacked, "sorrow" was applied to the opponent. Maybe it could be made to work in a certain radius around the unit?
By the way, the low (compared to other) damage values for level 7 units could be compensated for by a larger number of hit points, since the undead are resilient.
I'll think about this myself, but these are my observations so far.