Zeg was a Dalek inventor who conducted experiments to strengthen dalekanium, the alloy with composed the Dalek casings. He underwent an accident transformation which made him almost invulnerable.
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When Zeg's laboratory was struck by an Oquolloquox, the experiment he was conducting with artificial sunlight exploded and exposed him to radiation that bonded with his casing, creating a much stronger metal he called Metalert, which also turned his casing red in colour.
Although he was made stronger and more durable, his brain cells were affected so that he felt "joyful" and "dynamic". He challenged the Dalek Emperor for supremacy. The Brain Machine (a computer which the Daleks looked to for advice) decided Zeg and the Emperor would duel to lead the Daleks. Although Zeg survived acid and mercury, the Emperor destroyed him with liquid oxygen. Since he was resistant to heat, the extreme cold of the oxygen crushed him. (DC: Duel of the Daleks)