About mojko idea let me quote myself:
15. August, 2008
Time Warlock (rare)
Cost: 30 gems, 20 recruits
Attack = N where N is the actual turn number
Time Warrior (uncommon)
Cost: 10 gems, 10 recruits
attack = 10
if N > 50, wall +10;
if N > 100, production X2;
if N > 150, stock +5;
if N > 200, enemy tower -10;
if N = 249 or 250, instant tower victory;
where N is the current turn number
(the effects are comulative)